Hi
Apple Watch is not user-serviceable and Apple does not offer a screen repair or screen replacement service for Apple Watch.
Your Apple Watch will instead need to be replaced. It may be eligible for replacement via chargeable, out-of-warranty service or via AppleCare+, if previously purchased.
Contact Apple Support (mail-in service may be available, if required), make a Genius Bar reservation or visit an Apple Authorised Service Provider to establish your available service options:
Service pricing for the US, for example, is detailed here:
Apple Watch Service Pricing - Apple Support
In the meantime, note that Apple advises that you should not continue using (or trying to use) a damaged Apple Watch.