Hi
Apple does not offer a screen replacement service for Apple Watch. Your watch may instead be eligible for replacement via chargeable, out-of-warranty service - or via AppleCare+, if previously purchased.
Prices vary by model and region. Apple's out-of-warranty repair prices in the US, for example, are detailed here:
Apple Watch Service Pricing - Apple Support
Contact Apple Support (mail-in service may be available, if required), make a Genius Bar reservation or visit an Apple Authorised Service Provider to arrange for your Apple Watch to be inspected and to establish your available service options: