I cant pair my new iphone to my iwatch

You need to unpair the watch from your old iPhone first. Here are two ways to unpair your watch if you no longer have the paired iPhone:

• See the 'How to erase your Apple Watch if you don't have your iPhone' section of:

Unpair and erase your Apple Watch - Apple Support

• With a web browser, go to: https://www.iCloud.com and sign in with your Apple ID. Open Find My iPhone and select All Devices (at the top). Then click your Apple Watch, select 'Erase Apple Watch', and select Next until the device is erased. Finally click "X" next to remove your Apple Watch.

Then follow the steps in the article you came from.

What series watch is it? FWIW a lot of people have had trouble updating a series 3 watch. With the limited memory in a series 3 watch, your best option may be to make an appointment at the Genius Bar in an Apple Store near you and let them look at it:

Genius Bar Reservation and Apple Support Options - Apple

See the replies to:

Apple Watch 3 is stuck on preparing updat… - Apple Community