This how to assumes that you have already opened a gpx or kml file in GPX Viewer and you know how to display them on the screen.

Below are step-by-step instructions on how to reverse the direction of a track.

1. Open left main menu in GPX Viewer.
2. Tap on Track/route info button.
3. In the list of tracks and routes tap on the track you want to reverse.
4. Then scroll down below statistics.
5. Tap on the three dots on the right side.
6. Tap on Reverse direction button.
7. Confirm Yes in case dialogue that asks you to delete speed and time information from the track pops up.
8. Go back to the main map screen and the track/route direction will be changed.

Below is a video showing the instructions above: