In the app: For single routes, open the planned hike and tap More > Reverse. On the website: it’s under “Route options” in the right-hand column of the hike planner.

To keep a reversed version of an official hike, save your own copy of it.

One current limit: reversal works properly on single-stage hikes; multi-day hikes don’t reverse their days/stages yet (that’s in development — stages can be adjusted manually in the planner meanwhile).