If you have found a mistake, then there are a few things you can do:
Rate the solution. This will send a signal to our moderators, and help other students who come across the solution
Leave a comment on the incorrect solution. The author of the solution will be notified and they may correct their work (or explain why they think it is correct).
Add your own solution. Just log in, go to the exercise page, and click the big red "upload your solution" button. You can write your answer there.