how can i pair my watch from old phone to mew phone and also i forgot my watch passcode

If you’ve forgotten your Apple Watch passcode, you’ll need to erase it and set it up again. The iPhone it’s paired with will have stored a backup of the Apple Watch around the last time it was connected to the Apple Watch. The following support article can help you erase your Apple Watch: If you forgot your Apple Watch passcode - Apple Support

When you’re setting up your new iPhone, be sure to either transfer the data from your old iPhone to the new iPhone, or setup the new iPhone using a backup of the old iPhone. The following support article can help with this: Set up your iPhone or iPad - Apple Support

Once you’ve set up the new iPhone by transferring the data or using a backup from your old iPhone, you can pair the Apple Watch with your new iPhone. When setting up the new Apple Watch, you should be prompted to set it up using a backup. If you’re not prompted, your old iPhone may not have had a backup of your Apple Watch. The following support article can help you set up your Apple Watch: Set up your Apple Watch - Apple Support