On a classical rental, you have no specific action on Getaround to end a rental. You simply need to fill the rental agreement with the owner at the end of the rental, and sign it.
On an Getaround Connect rental, you have to return the vehicle to the location indicated in the vehicle details. Once you're parked, go to Your rentals > selected rental then click on "See on Getaround Connect". Click on "End the rental". You'll be guided step by step.
In any case, the rental ends automatically at the end of the half-day (midday for a morning rental, midnight for an afternoon rental).