In the left-hand pane, there are two buttons each for loops and cels: Add before and Add after. These will add a new loop or cel after or before the currently selected one.
By default, a newly added cel will be a copy of the currently selected one. In the Preferences dialog, this can be changed so that instead, new cels will just be filled with the current cel's transparent colour.
Click on the cel in bottom pane, and press DELETE
By removing all cels in a loop, the loop will be removed.