AFAICT, you *can't* rename a Mastodon (.social) account. You must delete that account and create another with the correct spelling. However, you might be hosed* if the e-mail account is the same as the one you used for the deleted account. 
*Mastodon says it sent a confirmation mail to the email address, but no actual mail arrives (not even in spam). So, you can't have the new account unless you also use a different email address.
Maybe you can use the same email address if you give it a few days rest and retry, but that points to a disconnect somewhere in the system.
Or maybe I'm just seeing things. 