Hello,
Just a heads up that this is a public community forum run by regular users like you, so none of us work for Microsoft and we cannot hop into your account or hand out free name changes.
To figure out how to get you out of this jam, I really need to know what exact version of Minecraft you are playing. Are you playing Java Edition on a computer, or are you on Bedrock Edition which uses an Xbox Gamertag? The way you change your username is completely different depending on which one you use.
Let me know which version it is so I can figure out what your options are.