This how to assumes that you have already opened gpx or kml file in GPX Viewer. If you haven't opened it yet here is How to open gpx or kml file.

Below are step by step instructions how to change icon for waypoint.

1. First off you need to start GPX Viewer.
2. Then open left main menu and click on Open files button.
3. If your device is running Android 6.0 or higher GPX Viewer will request permission to read and write external storage. Note that without you granting this permission to GPX Viewer, it won't be able to open gpx or kml files.
4. Next you need to navigate to directory in which your gpx file is stored.
5. Select your gpx file by tapping on it and then tap on Open button.
6. After this your gpx file will be opened and showed on the map

1. Tap on waypoint icon on the map.
2. After tapping on waypoint icon tap on bubble text shown above waypoint icon (if waypoint does not have any name or description, go to left main menu, then select Waypoint info option, and find your waypoint by listing it with < and > button in top bar).
3. Scroll little bit down and tap on “…” button.
4. From menu select Set waypoint icon option.
5. From shown dialog select Choose icon from library.
6. Choose some icon you like from the library and tap on it.
7. Go back to map screen and see your icon changed.

Below is video showing following instructions above: