Hello @Jpseph0,
The exact details will depend on which version, Android or iOS, of the app they have.
For the Android version:
Make sure to be in traveling mode by tapping "Profile" and then "Switch to Traveling".
Then tap "Trips".
Find the trip concerned and tap it.
Scroll down and tap "Change reservation".
You're then offered the facility to change the dates and the number of guests.
I don't have an Apple device but I presume that the iOS version has similar steps.
This is a duplicate post. This is the link to the other one:
https://community.withairbnb.com/t5/Help/State-extensions/m-p/668337#M153977
Steve