You can do it from within the device itself.
Here is how on a Nav VI, other devices will be the same or broadly similar.
Home Screen
Apps
Tracks
Find the track you want to convert
Tap on it
A map of the track will display. It can be quite hard to see due to its colour
Top left corner, tap on the three bars
Save as Trip
Chose which direction you want
Give it a name or use the one suggested
Click on Done
Device will calculate it for you
Device will tell you the route is now saved to the Trip Planner
Click OK
Back arrow to Trip Planner (or Home Screen)
Scroll down to find the freshly created route
Use as normal
Job done.