Depends what tools you have at your disposal.For me, load it into Garmin's Mapsource, double-click the track, and select the Invert option.Done.
The command should have the input file before the reverse filter. Otherwise it doesn't have anything to process.iegpsbabel -t -i gpx -f in.gpx -x reverse -o gpx -F out-reverse.gpxOr try using the GUI?
...Free ridewithGPS...
My choice would be:Bikehike.co.ukLoad RouteReverse RouteSave Route