Yesterday I managed to compile Ardour on Linux Mint and I noticed that it was version 6.9, even though I know there are later versions available.
What exactly did I do wrong? How do I update Ardour now after compiling it once? How do I keep myself up to date about changes to the software? 
You evidently downloaded the source code for version 6.9. You can fetch the source code for 8.6 here:
https://community.ardour.org/download?architecture=x86_64&type=source
There is no way to update a self-compiled version to a newer version. You will need to compile newer versions as they are released.
All right! Then how would you suggest I compile based on that? The way I did the compiling was using the Terminal and downloading it directly from git using git clone and the URL.
sudo apt-get install git
mkdir ardour
cd ardour
git clone git://git.ardour.org/ardour/ardour.git
cd ardour
./waf configure
./waf
sudo ./waf install
What would I need to modify to make it compile the 8.6 file I downloaded instead?
Not sure if this (waf install) works.
The directions you posted would get you the current git snapshot (More recent than the latest release and not always stable though ideally should be) and certainly not 6.9.
Because you posted the waf install step, I am guessing you are running a different version of Ardour installed on your computer, can be confirmed by running which ardour. To run the version you compiled by in the directory you ran ./waf and instead run ./gtk2_ardour/ardev which should run your compiled version.
Seablade
EDIT: Fixed ardev command per @x42 's post (Thanks for the reminder)
I figured it out! There was nothing wrong with my installation, except for the final step that I forgot to add.
ardour6
I needed to run that after doing ./waf install. When I switched to this it launched 8.6:
ardour8
Question, I tried launching Ardour 9, but it didn’t work. I suppose it isn’t available yet?
Also, thanks for the assistance! 
As seablade mentioned running waf install is highly discouraged, unless use a DESTDIR and package ardour.
The reason for this is that there is no clean way to uninstall Ardour. Your system may end up with a mix of different versions of Ardour, which can cause issues.
When compiling from source, ideally run Ardour from the source-tree:
./gtk2_ardour/ardev
(this will also allow to get properly annotated backtaces in case you encounter issues.)
Okay, but I am not familiar with in what context that should be run in. Do I need to replace the ./waf sections:
sudo apt-get install git
mkdir ardour
cd ardour
git clone git://git.ardour.org/ardour/ardour.git
cd ardour
./waf configure
./waf
sudo ./waf install
…with this?
sudo apt-get install git
mkdir ardour
cd ardour
git clone git://git.ardour.org/ardour/ardour.git
cd ardour
./gtk2_ardour/ardev configure
./gtk2_ardour/ardev
sudo ./gtk2_ardour/ardev install
I only installed Linux Mint last Wednesday, so excuse my lack of knowledge regarding this. 
I’ve never built ardour, but here’s what I see:
Looking at the README file, it says:
For information on building ardour:
https://ardour.org/development.html
Which seems to be the directions you followed, and then Robin is saying to stop prior to the waf install step. You just went too far.
It warns:
You do not need to install in order to use your new build of Ardour. You can run it from within the build tree:
Thanks for the help! Question, where do I find the executable in the folder to be able to make a desktop shortcut? Running it through the terminal every time seems a bit annoying to me.
At least when I used .waf I got an automatic shortcut and everything.
Never mind, I figured it out! What I basically needed to do was make a .txt-file and type in this code:
cd ardour/ardour
ardour8
Then I saved it as .sh and put it in the ardour folder where I saved it in Home/ardour. Once done I R clicked Ardour8.sh and went Properties → Permissions to activate “Allow executing file as program”.
After that I installed Alacarte with:
sudo apt install alacarte
Then I made a New Item and added the Ardour8.sh file executable and an icon I found inside …/ardour/ardour/gtk2_ardour/icons to make it look like a proper software:
After pressing OK I now have an executable shortcut of Ardour 8.
To modify this shortcut script for future releases I will just open the .sh-file in a text editor and replace “ardour8” with another number like “ardour9”, etc. The advantage of this is that I don’t need to install anything, just update the source files and reuse my shortcut script.
Are you sure about that? When I look for an executable file with that name from within the ardour directory it is not at the top level:
$ find ./ -name ardour8
./build/gtk2_ardour/ardour8
And as someone pointed out in an earlier post the script recommended in the build instructions to run from the build tree is ardev, not ardour8:
cd gtk2_ardour
./ardev
What does the output of “which ardour8” show when run from ardour/ardour directory? Is it possible you have a distribution version of ardour8 installed and it is actually launching that? Or have you moved the ardour8 script up two levels in the directory tree?
Given that waf install was run earlier in the thread, it may be the installed/compiled version that is being run, which as Robin mentioned above may cause issues down the line with future versions as there is no clean uninstall option like that.
Seablade
Well, the code works fine and it’s installed in my Home folder, which is good enough for me. I don’t need to access gtk2_ardour nor ardev to run the software.
Just running ardour8 or some other number in the code launches the correct version from the source code. You can even open older versions by typing using ardour6. Once you’ve accessed the ardour folder with the source code you can just run it with no issues.
I completely cleaned out the folder where I installed it and just downloaded the source code without compiling, so unless there are other folders out there where files are located there is nothing else left from my waf install. I don’t need to do anything else except update the source code from what I’ve seen.
Again this is where which ardour8 will tell you exactly where the executable you are running is located.
Seablade
Got this from that command:
/usr/local/bin/ardour8
When I typed in other numbers than 8 nothing came up.
That is likely a leftover from running ./waf install
Okay! Are there any other folders I should look in beyond deleting that specific folder?
./waf uninstall will do something.
The reason why it is not a good general solution is that it will only remove the files that this particular version of the source code would have installed. Which may be too much, not enough or just right.
Yeah, that broke my shortcut. So I guess I need to rewrite it to make it work again, hehe. 