I had originally specified the 29th of Feb but when I met with the sitter we decided to have a trial day on Feb 26 instead. So I would like to change from the 29 th to the 26.
There are several ways to change/modify a booking. Fastest, probably to just call Rover Customer Support at [Edit: Rover’s contact options have changed. Visit the Rover Help Center at https://support.rover.com/ to find the phone number, help articles, or chat with the team] or ask the sitter to do so BEFORE he/she accepts the booking. Your sitter should know how to do this.
Other method: Just create a new one for that date. The second one wouldn't be charged to you unless the sitter formally accepted it and then you subsequently paid. Even if the sitter accepts, you don't have to pay.
Add: I think the fastest would be the last method of just creating a new one and ignoring the second so long as you haven't already paid for it.