On a classical rental, you have no specific action on Drivy 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 Open rental, you have to return the vehicle where you collected it at the beginning of the rental. Once you're parked, go to Your rentals > selected rental then click on "See on Open". 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).