@Len5, what exactly do you mean with "pending" reservation: have you accepted it and Airbnb has confirmed, yes, then a cancellation on your part carries severe penalties, and they are much worse for 2017 then what is now in place, up to $500 actually, and your place would remain blocked for those dates also. Don't cancel!
But if you are at the stage that the reservation is pending becuse you have not accepted it yet, you have the option to decline (that is not cancelling) - no harm done! then change your pricing and ask the guest to submit a new request if they still like to stay with you.
or you can simply use the "change reservation" option and figure out the new total price, but you must include your cleaning fee in that subtotal. That is what I do when guests change their length of stay. simple and straight forward. In case yours is a confirmed reservation, you could try "change the reservation" also, but if guest doesn't agree to it, it reverts back to the original booking. depending on how much $$ is at stake, it might not be worth it to make the guest feel uncomfortable about it....