If you need to modify a reservation before it begins, you can update the booking up until the booking start time. If your booking has already started, the host can add the additional hours as overtime after the booking. Hosts can request overtime in the Peerspace app (open the booking's message thread in your Inbox and tap "Request additional payment from guest") from when the booking starts until 72 hours after it ends. You'll have 48 hours to accept and pay, or dispute the request.
Please note: overtime is entirely dependent on host availability. There is no guarantee you will be able to use a space longer than initially booked.