As of today, we unfortunately do not have a feature in place to extend an ongoing rental period. In order to extend, you will have to send a new rental request for the extra days. When the owner has approved your new request you pay to book the extended period the same way you did for your first rental period.
We are working on a solution that enables the extension of rental periods so that the whole process becomes much simpler and smoother. We hope to have that solution in place soon.