When you have rented something then there is nothing you need to do in order to end the rental period after you have returned the thing you rented. The rental period will automatically end when the rental period is over or when the owner marks the thing as returned. When the rental period is completed, you will be asked to leave a review.
If the owner forgets to mark the thing as returned, he will be reminded to do so. There is nothing you need to worry about. The owner needs to mark it as complete to get paid (if it was marked as handed out at the beginning of the rental period). You will not be charged with more rent even if the owner is late with finishing the rental period.