How do I change my AppleID

See Change your Apple ID https://support.apple.com/en-us/HT202667. 

And, here is What to do after you change your Apple ID or Password https://support.apple.com/en-us/HT204071  (supplementary to what the fist link says)

And FYI, see iCloud: About your @icloud.com, @me.com, and @mac.com email addresses  https://support.apple.com/en-us/HT201771

Post back if you have questions about what you can do if your Apple ID (1) is a third-party apple ID or (2) ends in icloud.com, me.com, or mac.com. 

You can edit your personal information by logging into your AppleID at https://appleid.apple.com

See the article donv posted.