Back up both iPhones
Sign out of Apple ID on the old iPhone
Turn off Find My iPhone on the old iPhone
Erase all content and settings on the old iPhone
Remove the SIM card from the old iPhone
Insert the SIM card into the new iPhone if compatible
Turn on the new iPhone
Follow the setup steps on the new iPhone
Restore from iCloud backup or transfer from the old iPhone
Sign in with your Apple ID on the new iPhone
Check messages, calls, photos, apps, and settings
Remove the old iPhone from your Apple ID device list
Contact your carrier if you need SIM or eSIM activation