How do I uninstall a program installed with the Appimage Launcher?

So basically I installed a program with AppImage in Linux Elementary OS Loki, but now I have no idea how to uninstall it, and I didn't find anything in the documentation of AppImage.

Since an AppImage is not "installed", you don't need to "uninstall" it. Just delete the AppImage file and the application is gone. Additionally you may want to remove menu entry by deleting the desktop file from $HOME/.local/share/applications/.

Files and directories with names starting with a full stop (dot) (.example) are hidden - you might need to turn hidden files visible. You can probably find it somewhere in the settings of the file manager you use or in many file managers you can do that with ctrl+h.

Everyone here is correct, Appimage's aren't installed. In fact that is kind of the beauty of them. They are self contained and compressed. I think they fail to elaborate a little on the different possibilities either distros, or users can encounter though.

If you are using something called appimaged the deamon for appimages. It can make it seem like the file is actually installed. It manages them by reading them in specific places and creates file associations, menu entries, and many other things. Some applications may not want to be dependent on the deamon running since it is technically optional. In this case they may write ".desktop" files, or similar depending on DE to your menus directory.

To read more about the deamon in case you encounter this situation you can find out more by going to the link below. https://github.com/AppImage/appimaged/

Usually in Debian based distros you can look in

/home/$USER/.local/share/applications

So once you delete the appimage, you can also delete the menu entries you find in that directory.

Also you should read more about appimages in their documentation. https://docs.appimage.org/introduction/quickstart.html

The quick star is short and concise, but you can find out more if you explore the docs.