You can access the Manage Content options via the icon in the top right area.
You will be presented with the following options
Add a single game
You can add one game from this option.
Add folder for one system
If you have a bunch of games for one system, you can add them in one go.
You can rescan all or individual systems from this option
You can manage local or network storage from this option.
From this option you can delete individual or all the games in your game library. You can find a more detailed guide on how to uninstall games here.
You can find in-app help from this option.