DO NOT change the name if the main Admin account in a Mac. And be certain you have a trusted local disk backup before you proceed with any of this.
I say this because we read too many sad stories from users who ALMOST followed the complicated procedure correctly, but made one teeny-tiny mistake, and instead were locked out of their Mac with no recourse except to ERASE and start over.
Instead, create a NEW Admin account, with a name and password you prefer, and use to accomplish your goals.
One way to proceed with the new account is to copy your files to the new account, and use it for a week or two to be certain you copied everything. Then if it annoys you, you can reclaim a little space by deleting the old Admin account.
Some users prefer to use a non-Admin account as their daily account. When you do this, installing new things is less likely to happen by accident, because you will be required to supply the Admin account-name AND Admin password, and extra layer of protection.
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NB>> If you are stuck with the account because your computer was used by someone else first, and was set up with their Apple-ID, you may get a Big Surprise when you go to do any software upgrades. You will need the previous User's active Apple-account-name and password to do any upgrades.
The correct procedure to follow when starting to use a pre-owned computer is this one:
What to do before you sell, give away, trade in, or recycle your Mac - Apple Support
...and that will allow you to set your own Mac account names and passwords in the process.
