You can do this by uploading the file to something like Garmin Basecamp (Free download and maps are available free from http://garmin.openstreetmap.nl/) and trimming off the bit you don’t want. Also, online there are quite a few packages, this one seems good, but I have limited experience with it. https://gpxstudio.github.io/ or https://www.gpxeditor.co.uk/
I am not aware of a method on SA.