replacing new watch with existing
How do I replace old watch with new SE
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If you want to rep[lace the old watch, first backup and unpair the old watch:
Back up your Apple Watch - Apple Support
Unpair and erase your Apple Watch - Apple SupportSet up your Apple Watch - Apple Support
Then see step 4 in:
Set up your Apple Watch - Apple Support
PS - you can have both watches paired to the same iPhone. You just can't wear both at the same time.
Set up more than one Apple Watch - Apple Support