You have a new iPhone 12, but you had to lay out some serious moolah for it. If you didn’t trade in your previous iPhone with Apple, you may want to sell it to someone else. But don’t do this wit preparing your smartphone for selling or giving it away.
Assuming that all your data has been backed up and moved to your iPhone 12 (and don’t proceed if it hasn’t been transferred), take the steps:
If you paired an Apple Watch with your iPhone, unpair your Apple Watch. Open the Watch app on your iPhone. Go to the My Watch tab and tap All Watches.
Tap the info button next to the watch that you want to unpair. Tap Unpair Apple Watch.
Sign out of iCloud and the iTunes & App Store. If you’re using iOS 10.3 or later, tap Settings > [your name]. Scroll down and tap Sign Out. Enter your Apple ID password and tap Turn Off.
If you’re using iOS 10.2 or earlier, tap Settings > iCloud > Sign Out. Tap Sign Out again, then tap Delete from My [device] and enter your Apple ID password. Then go to Settings > iTunes & App Store > Apple ID > Sign Out.
Go back to Settings and tap General > Reset > Erase All Content and Settings. If you turned on Find My [device], you might need to enter your Apple ID and password.
If asked for your device passcode or Restrictions passcode, enter it. Then tap Erase [device].
If you’re switching to a non-Apple phone, deregister iMessage. If you transferred your SIM card from your iPhone to a non-Apple phone, put it back in your iPhone.
Make sure that you’re connected to your cellular data network.
Tap Settings > Messages and turn off iMessage.
Contact your carrier for help transferring service to a new owner. If you aren’t using a SIM card with your device, you can contact them to get help transferring service to the new owner.
Now remove your old device from your list of trusted devices. You can view and manage a list of your trusted devices on iOS, macOS, and in the Devices section of your Apple ID account page.
(This how-to is based on my experiences and info on Apple’s support pages — where the images sometimes come from.)