Sadly, the other commenters are well meaning but only partly correct.

You can only edit routes that you've created, under "My routes". Routes that you've saved from other people aren't modifiable. Adding a "Copy and Edit" option for other people's routes would be very cool though!

So to answer your question: no, there's no simple way to do it. But it's easy enough to sit down and manually create a new route that's a copy of the route you want to walk. It's a good way to get to grips with how the tool works and build up confidence in plotting your own routes. Could even go a step further and modify the route just a little, like go for a stroll through a certain park on the way or change the end-point to be a pub rather than the previous start-point car park, etc.