Use Open GPX to bring an existing trace into the editor. Before changing it, check that you have the intended circuit layout and that the line is in the right place.
Look for unwanted connecting sections, duplicate points or a start location that makes the lap awkward to work with. Keep the original GPX separately so you can return to it.
If import fails, post the exact error, browser and a description of where the file came from. File attachments are not enabled in this forum; do not paste a very large GPX into a reply.