Absolutely you can cancel. Depending on when she is scheduled for and the sitters cancelation policy you may end up being charged a cancelation fee. (If the booking is at least a week out, you can get a full refund.)
Through the app there should be a section to modify the booking, in which you can cancel your booking. Or you can contact Rover Support at 188- 453-7889. (They are very friendly and helpful and would be able to help you cancel your sitter. They are also able to help you find another sitter, if need be.)
Your not alone, I am sure many of us have been to quick at the jump but In the future, I would recommend not to confirm a stay until after your Meet and Greet. You also should scheduling a few Meet and Greets until you find the perfect match. You want to make sure that you are 100% comfortable with your sitter, otherwise you will end up worrying about your dog your whole trip. =(
Best of Luck!!
-