You have to make one pit stop during the race, which must not be in the first or last lap. Here’s a short silent video of the procedure. Your car will be repaired after the pit stop:
If you need your car repaired a second or third time, tell the pit watcher in advance, he’ll try his best to shoot your car, so the wrench icons work for yuo, there’s no need to wait 10 seconds for your 2nd, 3rd etc. pit stop.