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 or other Apple policy, then your 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 (not "iwatch") to be inspected and to establish your available service options:
- US: Apple Watch Service Answer Center - Apple Support
- Elsewhere: 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.