Step 1:

Click on Rainbow Six Siege inside your library.



Step 2:

Hover over the gear icon.



Step 3:

Hover over 'manage'.



Step 4:

Click 'uninstall'.



Step 5:

Click 'Uninstall' and relinquish yourself from this life destroyer of a game.



Step 6:

Finally take the first shower you've had in months and touch some grass.