I want to change my email address

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

And see 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. 

Where do you want to change it? You can go to the Gmail web site and get a new address. If that isn't what you want, more information would help.