Changing Apple id

Apple ID and Password

Change your Apple ID

See the instructions below.

Change your Apple ID (Be very aware of the restrictions here.) https://support.apple.com/en-us/HT202667

Before changing your Apple ID using a computer, sign out of all Apple  services that use your Apple ID. If you are going to use an iPhone, iPad, or iPod, then stay signed in with your Apple ID on only that device. This instruction is taken from the above document under the heading Before you change your Apple ID.

For sign out purposes, see Where can I use my Apple ID? https://support.apple.com/en-us/HT202659 “This list is provided for informational purposes only and is subject to change without notice. It's not intended to include every feature or service that uses Apple ID. Visit the Apple ID support page for more information.”

There may be some redundancy, but see What to do after you change your Apple ID or password 

 https://support.apple.com/en-us/HT204071

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See Use your mobile phone number as your Apple ID https://support.apple.com/en-us/HT207944#change-appleid See the heading Change your Apple ID to a different mobile phone number

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Use a passcode with your iPhone, iPad, or iPod touch

https://support.apple.com/en-us/HT204060#adjust

If the above doesn't help, what happens when you try? Error message(s)?