An extension works exactly the same as a rental request. In the booking overview, the renter can send a rental request. This request has a new end date and time for the current booking. If the owner agrees and the car is available, they accept the request. The renter can then pay the extension, finalising the new rental end date and time.
The owner has the choice to accept or decline the extension request and inform the renter. When denied, the rental period will end on the original date and time.
Note that if you do not accept the extension request 1 hour before the original end time, the rental request will be automatically denied. It's possible to extend the same booking multiple times.