@Stella235
You can change the reservation by changing amount of guests. Price will be calculated automatically if your listing charges a fee for those extra guests. But when reservation is allready started it probably will not work. Then you can use the resolution centre to ask for extra money, but only if your listing normally charges also for extra guests at time of booking . If none is working for you, ask the guests to message you details of the extra guests and date of arrival. Then calculate total new price of reservation manually, change the reservation by put it the new price. Guests will have to accept and will pay the price difference to Airbnb. Never accept payment off site (only trhough Airbnb).