How to change Apple ID user

There's some nomenclature confusion here. An Apple ID "username" is actually an email address. If that is what you mean, refer to: Change your Apple ID

1. Go to https://appleid.apple.com and sign in.

2. In the Sign-In and Security section, choose Apple ID.

3. Enter the email address that you want to use as your Apple ID.

4. Choose Change Apple ID.

5. If you changed your Apple ID to a third-party email address, check your email for a verification code, then enter the code. Learn what to do if you didn't get the email. --> If you didn't receive your verification or reset email - Apple Support

6. If you use services like iCloud or Messages to share with friends and colleagues, sign in to those services with your updated Apple ID. --> What to do after you change your Apple ID or password - Apple Support

If you mean the identifying name on your account such as "George":

1. Go to https://appleid.apple.com/

2. Authenticate

3. Click Personal Information

4. Click Name

The article that you read that shows on the top of the page clearly shows how to change your Apple ID. Is there an issue you are having following the procedures in that Support Article?