I want to change my Apple ID
How do I change my Apple ID on my phone? Every time I try to change it, it says I can’t. I don’t know what I’m doing wrong.
See this guide:
Change your Apple ID - Apple Support
if you mean you want to use another - already existing - AppleID you need to completely wipe to current AppleID from your phone …
… and then set it up as the “other” AppleID.
See: What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support