You will have to visit an Apple Store or Authorised Apple Service Provider and let them check the watch, or contact the official Apple Support about it.
But, in most cases, Apple will not replace the watch for "Cosmetic" damage, only if the manufacturing process is the reason.
Apple - Find Locations
Contact Apple Support - Apple Support
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This Warranty does not apply: (a) to consumable parts, such as batteries or protective coatings that are designed to diminish over time, unless failure has occurred due to a defect in materials or workmanship; (b) to cosmetic damage, including but not limited to scratches, dents and broken plastic on ports unless failure has occurred due to a defect in materials or workmanship;
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copied from: Legal - Apple Watch Warranty US - Apple