@Maria545 As others have said, it's really up to you to modify their booking, after which they can "accept" the change. The reason that it's more on you than them is multi-fold: If they cancel the original booking and then rebook for longer, they'll pay the original AirBnB guest service fee and then the new fee (partially duplicating the old fee). Also, if they don't cancel but simply add the add'l days as a second booking, they'll get charged twice for your cleaning fee (if any). And so on.
It's quite easy for you to modify their booking. Once you do that, they are left with the option to Accept/Decline the new offer.