Hi
Contact Apple Support (mail-in service may be available) or make a Genius Bar reservation to arrange for your watch to be inspected and to establish your available service options, likely timescales and cost:
- Contact - Official Apple Support
- https://twitter.com/applesupport
- http://www.apple.com/retail/geniusbar/
Repair service costs vary, depending on your location and Apple Watch model. Prices in the US, for example, are available here: