At anytime you can easily move the position of an existing geofence.
Simply click on the Geofence (you'll see a ✋icon) and then drag to the new position. Once this is done you'll want to Save the changes. Your changes will be available for users after the next Rule Download (What's a Rule Download?)*.
*If you're testing, simple swipe closed, and re-open the app you're testing with. This will cause the SDK to re-authenticate and force a Rule Download to occur.