A post of mine has been locked. There was an error in the answer I gave, which was pointed out, and I wanted to correct it, but it got locked.
How do I unlock it so I can fix it?
What is the purpose of the lock?
You cannot unlock a post. The very point of locking a post is to prevent users (including the owner) from altering it.
See What is a "locked" post? for further details.
You can contact a moderator via a flag or ask in the Math mods' office chat to get further details and to inquire under what conditions the post could be unlocked.
For the specific case, for some reason you altered the post, like, ten times. This creates noise. You now have some time to decide what you actually want to do. Once you figured that out, contact one of us, as indicated above.