how can i change the dates requested?

That depends on what stage you are at. If the request hasn't been confirmed yet, you can edit the dates. Go to the page corresponding to the stay request (Inbox -> Pending Requests -> click on the specific request). Click on the Edit button that is just to the right of the dates listed at the top of the black box in upper left of page, and a separate window should appear that lets you change the dates.

If the stay has already been accepted and confirmed by both parties, you have a couple of choices. You can call Rover Support and ask them to change the dates, or you can cancel the stay and set up a new stay for the revised dates.