How to erase everything from old iPhone and disconnect old phone from new one.?

I cracked my old iPhone and got a replacement. When I turned on the new iPhone all of the data from my old iPhone synced onto my new one. I’d like to know how I can erase everything from my old iPhone, without everything being erased from my new one. As of now, I’ve noticed that if I delete my notes on either... show more I cracked my old iPhone and got a replacement. When I turned on the new iPhone all of the data from my old iPhone synced onto my new one.

I’d like to know how I can erase everything from my old iPhone, without everything being erased from my new one.

As of now, I’ve noticed that if I delete my notes on either device, then it will be deleted on the other one.

I have to send my old phone back, so I just need help with how to disconnect them and erase everything from my old one.
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