Update: Is it possible to un-claim the certificate or change the solution links?
You need to go to the page where you submitted the link, paste the new link, then click “I’ve completed the challenge” and “Submit and go to the next challenge”.
I also cannot change the solutions links after getting the certificate. I didn’t know how CodePen works, so I just pasted empty CodePen pages, thinking it should autosave, now my solutions link to the same empty CodePen page.