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Whoever is going to change should log out of the current

AppleID on their device, create a new AppleID and use that

to log into their device.

Use this link to create a new AppleID:

How to create a new Apple ID - Apple Support

The person changing ID should follow the steps in this

link to be sure their device is ready for a new AppleID:

before sell/give away iDevice: What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

You may wish to look into Family Sharing so both can

access previous purchases, etc;

Family Sharing - Apple

Do you both use the same AppleID?

If so, you each need your own AppleID?

Are you sharing a phone number or email address?

Stop sharing an Apple ID / iCloud account.