Apple Watch how do I pair the replacement

Hi

If you have not yet unpaired your original Apple Watch from your iPhone, then follow the instructions here:

Unpair your Apple Watch and iPhone - Apple Support

To pair and set up your replacement Apple Watch, follow the instructions here, starting by updating your iPhone to the latest version of iOS (be sure to make a backup first) and selecting to Restore from Backup if offered the option during setup:

Set up your Apple Watch - Apple Support