Hi
Apple advises that you should not continue using a damaged Apple Watch.
Apple Watch is not user-serviceable and Apple does not offer a screen repair or screen replacement service for Apple Watch.
If the issue is not covered under warranty (due to being defective), then your Apple Watch may instead 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 arrange for your Apple Watch to be inspected and to establish your available service options: