I don't know of a way to do it on the fly from the device itself, but if you use the Garmin Connect website, you can do it there. You can import the route to Garmin Connect from RWGPS, or convert an activity that you've already done into a course. Open the course in Garmin Connect, and in the details section, click on the three dots next to where it says "Send to Device". Click Edit and there will be an option to Reverse Direction.

This works better on a computer. On a phone I don't think you can do it from the mobile site; you need to use the desktop site and it's kind of a clunky process so it's best if you can do it ahead of time.