Hi
Unless the issue is due to a manufacturing defect, cosmetic damage is not covered under either Apple's Limited Warranty or AppleCare+.
If the issue is not covered under warranty, then your Apple Watch may be eligible for replacement via chargeable, out-of-warranty service.
I suggest that you contact Apple Support (mail-in service may be available), make a Genius Bar reservation or visit an Apple Authorised Service Provider with a view to having your Apple Watch inspected and to establish any available service options, likely timescales and cost:
- US: Apple Watch Service Answer Center - Apple Support
- Elsewhere: Select your country for Apple Watch service and repair information
To avoid further damaging the casing and to ensure that your Apple Watch maintains all functionality, Apple advises that you should not polish or buff your Apple Watch with abrasives.