As of today, we unfortunately do not have a function to extend an ongoing rental period. In order to extend, the customer will have to send a new rental request for the extra days.
If, on the other hand, you have agreed that the customer can keep the thing for a day or two extra, at no extra cost, then you do not need to extend the rental period. Then you just wait until the customer has returned the thing before marking it as returned.
We are working on a solution that will allow extension of rental periods further down the line and we will inform all users of this feature when it is available.