What exactly are you trying to achieve?
Remember that "routes" are never saved, they are always created on-the-fly, just using a start-point and a destination.
In other words, the unit cannot remember how it got somewhere, only where it started from and went to.
If all you want to do is go back to where you started, then there are several methods, but there isn't a single button to "reverse" the last route.
If you have your starting point saved (as a favourite, or a POI, or as your Home location for example) then you can just "Navigate to... back to it.
If you have already travelled to and from certain places at some time in the past, then you can often find all the places you want in the "recent destinations" list.
If you ever use the "Itinerary" feature (that has multiple destinations or waypoints) then you can use the menu to manually re-order the steps in the opposite order (there was even a third-party add-on called "Reverse Itn" (
HERE) that did it for you, but it's quite a hassle to install.