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.