To customize these buttons, you should open the Sidebar, then navigate to "Account/Settings" and from there to go "Settings". Once there, you'll find the "Swipe Buttons" option under "Customize Buttons/Actions".

If you want to customize buttons that appear when you swipe on a task in the main view, tap on "Tasks". Then tap on the plus icon on the top-right and you'll be able to choose from the variety of actions.