You can do it on a 550 but it's not perfect.
1. If you created a route using a number of way or viapoints, you can reverse these manually by moving them up and down the list manually. It's laborious but it works. Do though take care of any points placed on one way streets and the like.
2. You can display the path you rode as a track and then reverse it. You cannot though save it, you'll just have to follow the magenta line, with no voice prompts.
3. You could simply ask the device to plot you a route from B to A (as opposed to A to B) and then pull it around manually to match that which you took when you rode A to B.