Hi @Maria369,
A) as long as your pre-approval has not yet been accepted you can change the dates as follows:
1) Retrieve the pre-approval first
2) Then just send out a new pre-approval with the adjusted dates
3) the guest needs to accept within 24 hours, otherwise it will expire automatically
B) once the pre-approval has been accepted you only can change the resarvation as follows:
1) Go to My Reservations
2) Cancel or Change
3) click on Change and override the original dates with the new dates, the price then will automatically be adjusted
4) the guest needs to accept your change request
Hope this helps
Ralf