Run the following commands in your preferred terminal:
sudo apt install ./path/to/planet/package.deb
Now you will need Minecraft Pi: Reborn to complete the process of installing Planet. You can get Minecraft Pi: Reborn at https://mcpirevival.tk.
Once you have downloaded Minecraft Pi: Reborn, you should open Planet.
When Planet opens, it will ask you for the executable of Minecraft Pi: Reborn that you have downloaded. Press the "AppImage" button and select the file you have downloaded.
Planet now should be ready to launch! If you are having problems, join our Discord: https://dsc.gg/mcpiscript