Change @icloud

Note the restrictions in the article.

Apple ID Name Change

 What to do after you change your Apple ID or password - Apple Support     I suggest doing this before you change your Apple ID or password.

I had entered @gmail.com in apple id

i thought to change @icloud.com

so how to change it

help me

Apple ID Name Change

What to do after you change your Apple ID or password - Apple Support

Did you read the article in the first link you posted?