How do I change my Apple ID to a different 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)

Note that you will not be able to change to your gmail address if your current Apple ID ends in mac.com, me.com, or iCloud.com.

If you change your Apple ID, then anything you have gotten from the Online Stores will not transfer to the new Apple ID. You will need to repurchase and re-download it all.

For Problems With Your Apple ID

1. Forgot iCloud Password
2. Forgot Apple ID
3. iForgot Password - Apple ID
4. iForgot UserName - Apple ID
5. Apple ID Password Change
6. Apple ID Name Change
7. Apple ID - Apple Support
8. Manage your Apple ID primary, rescue, alternate, and notification email addresses
9. About your Apple ID email addresses
10. Get help with Apple ID
11. Contact Apple for help with Apple ID account security - Apple Support
12. What to do after you change your Apple ID email address or password - Apple Support

Reset Security Questions

Contact Apple for help with Apple ID account security

If you forgot the answers to your Apple ID security questions - Apple Support

You may need to reach Apple’s Account Security department. If you cannot find your country or are not able to call, then complete this form and send it to them.

Try using Apple's iForgot service. If it says they don't have sufficient information, you need to contact Apple's Account Security team. To reach them, click here and pick a method; if you can't use any of the ones listed, fill out and submit this form.