How do I change my Apple ID on my iPhone?
Got a new iPhone but it would not let me log in with my existing Apple ID and made me create a new ID. How do I switch to my existing ID?
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If you don't have a lot of data on the phone, I suggest you use the article below to completely remove the new ID.
If necessary, sign out of the new ID and in with the old ID using the article below as a guide.
What to do after you change your Apple ID or password - Apple Support.