Hi
Apple does not offer a screen repair or screen replacement service for Apple Watch and Apple Watch is not user-serviceable. Your watch will instead need to be replaced.
Apple's Limited Warranty covers issues due to defects in materials and workmanship, but not due to accidental damage. If you have previously purchased AppleCare+, then the issue may be covered as one of up to two claims for accidental damage, each subject to a service fee / excess.
Otherwise, your Apple Watch may be eligible for replacement via chargeable, out-of-warranty service. Out-of-warranty repair service prices vary, depending on your location and model.
Contact Apple Support (mail-in service may be available, if required), make a Genius Bar reservation or visit an Apple Authorised Service Provider in a country where Apple offers service for Apple Watch to arrange for your Apple Watch to be inspected and to establish your available service options:
Select your country for Apple Watch service and repair information
In the meantime, note that Apple advises that you should not continue using a damaged Apple Watch.