In reply to Sam Mayfield:
Those lenses in Malk's link are very bad quality. Not worth the effort to change them. A 'proper' lens will set you back close to 100 quid.
If you want to go that way, the best thing to do is to buy a non-working gopro with a lens in good nick (search ebay for 'spares' or 'repair' gopros) and take out the lens. Advantage of that is that you get to break apart the non-working camera before you do so with the good one, so you get to practice without risking yours.
Check the compatibility of lenses across models before buying though.
Good luck