Service
How can I switch my service to another Iphone
iPhone 6, iOS 12
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If you are referring to Cellular service then simply change the SIM
If you are referring to iCloud Service, use the same Apple ID or use Family Sharing
You can use Family Sharing on your iPhone, iPad, or iPod touch with iOS 8 or later, your Mac with OS X Yosemite or later and iTunes 12, or your PC with iCloud for Windows. On your iPhone, iPad, or iPod touch, tap Settings > [your name]. Then tap Set Up Family Sharing > GetStarted, then follow the onscreen steps.
Family Sharing - Apple Support
https://support.apple.com/en-in/HT201060