Open the Settings app
Tap your name at the top
Scroll down and tap Sign Out
Enter your Apple ID password if prompted
Choose the data you want to keep on the iPhone
Tap Sign Out again to confirm
Return to the Settings app
Tap Sign in to your iPhone
Enter the new Apple ID and password
Follow the on-screen verification steps
Enter the device passcode if requested
Review iCloud, App Store, and other account settings
Open the App Store and sign in with the new Apple ID if needed
Check Messages, FaceTime, and iCloud to confirm the new account is active