Press the “Windows + R” shortcut to launch the RUN utility. After that, type lusrmgr.msc and press Enter. This will launch the Local Users and Groups console in Windows 11.

In the Local Users and Groups console, you will see the list of all user accounts. This list includes the default user accounts (Administrator, DefaultAccount, Guest, and WDAGUtilityAccount) and your account.

to rename or change the username or account name of your account in Windows 11, right-click on it and select Rename.

After that, type the new username and press Enter.

After changing the name, it’s time to change the full name of your account in Windows 11. So, right-click on your account and select Properties.

Now, in the properties window, type the new full name for your account. After that, click Apply and OK to save the changes.

Now restart your Computer.

If the above steps do not help, create another admin account, transfer your files and delete this user.

Settings (Win+I) > Accounts > Other users > Add account.

After adding a new account, change the account type to Administrator.

From https://learn.microsoft.com/en-us/answers/questions/4136863/why-am-i-unable-to-change-my-username-in-windows-1