renaming my email domain/messages won't receive

I'd like to edit my email domain to read @sfcllc.org rather than @summerfieldsconsulting.org. how do I edit this? will I have to purchase a new domain?

also, my messages will send but I can't receive. If I send a test email to my zoho email account, that message won't even give me an error. It just disappears into space.

Someone please advise.