I got a new phone yestorday. My apple watch series 5 won't connect to the new phone

Follow the steps in the ;'If you don't have your old iPhone or you erased it' section of:

How to pair your Apple Watch with a new iPhone - Apple Support

Note that step 1 will, direct you to the 'How to erase your Apple Watch if you don't have your iPhone' section of:

Unpair and erase your Apple Watch - Apple Support

You can also try:

• 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.