Course challenge restart

I started a course challenge a long time ago and got mostly through it, more than 50% wrong. now i've done all the kahn academy units and stuff for it and the last thing I need to do is the course challenge. but there is no restart. and I know that if I just skip the questions or continue as I was it will lower down all my hard work :(. i wish you could restart. if there is a way to do so please tell me!