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See this article Change the name of your macOS user account and home folder - Apple Support

Read and understand the article cited. Then DO NOT DO IT.

Instead, create a new user Account with the name you desire, and copy your files over to it.

WHY:

Too many users have made a teeny-tiny mistake in re-naming their Account, and locked themselves out of their Mac, unable to log in at all. Without a Trusted Backup, all their files might be lost forever.