Sign out of your Apple ID on the device you want to change
Go to Settings
Tap your name
Scroll down and tap Sign Out
Enter your Apple ID password if prompted
Choose whether to keep a copy of your data, then tap Sign Out
Sign in with the new Apple ID
Go back to Settings
Tap Sign in to your iPhone (or iPad/Mac)
Enter the new Apple ID and password
Follow the on-screen prompts to complete verification
If you use iCloud, confirm iCloud settings (contacts, photos, iCloud Drive, etc.)
If prompted, verify your identity using the trusted phone number or device
Update your Apple services account if needed (App Store, iMessage, FaceTime)
On iPhone/iPad: Settings > Messages > Send & Receive, then select the new Apple ID
On iPhone/iPad: Settings > FaceTime, then ensure the new Apple ID is selected
On iPhone/iPad: Settings > Apple ID > Media & Purchases, then update App Store and subscriptions
If you need to change the Apple ID used for an existing subscription, cancel and resubscribe under the new Apple ID (if required by the service)
On Mac: System Settings > Apple ID > Overview > Sign Out, then sign in with the new Apple ID
On Apple TV: Settings > Users and Accounts > [your account] > Sign Out, then sign in with the new Apple ID
On Windows (iCloud for Windows): Open iCloud for Windows > Sign out, then sign in with the new Apple ID