Appimages are portable pkgs, meaning you don't need to install them, just make them executable and run. You can do this in 3 ways:
1. Using the file manager:
Open your file manager and navigate to the appimage location, in your case
Downloads, select it and hit
Alt + Enter, this will open the properties' dialog, click on the
permissions tab and check the box that reads
make this file executable or some similar wording, double-click the app to launch it. Some appimages will offer to integrate with the system, some will not, if your appimage doesn't, you can do it easily, now I don't know which DE your distro uses, but most have an option to edit the applications' launcher menu, so you can add apps to it and they are available, so check whether yours has such an option.
2. Using appimagelauncher:
Download appimagelauncher from its GitHub repo
https://github.com/TheAssassin/AppImageLauncher/releases there are different formats for you to choose, you might want to use the
.deb one.
Install it in your system with
Code:
sudo apt install ./appimagelauncher_2.2.0-travis995.0f91801.bionic_amd64.deb
I haven't used it but according to the instructions on that page, you just need to double-click an appimage for it to do its magic.
3. Using the terminal:
Launch the terminal and
cd to the directory
make the appimage executable
Code:
chmod +x pkg_name.AppImage
launch the appimage
Launching the appimage from terminal, at least for the very first time, might be useful to check for errors, so I do recommend you do this, once you make sure there are no serious errors that may affect the appimage's functionality, then proceed to integrate it with your system, so you can launch it the same way you do with the rest of the apps, that is, via the DE's menu.
This is a must-read if you plan on using appimages
https://docs.appimage.org/user-guide/index.html