@David3696 . The first thing I do is the math. How much should your guest pay for both the original reservation plus the extra guests? Then I would go to "change or cancel" this reservation. Change the reservation to the new number of guests [use the maximum night count] to ensure that AirBNB will "back you up." Now manually change the dollar amount to the calculation you made in step 1.
Tedious, but it works and you will know that the amounts are correct. [AirBNB's math at the change screen is often incorrect.]