when to get battery replaced in apple watch
How to know if you need a new battery for apple watch
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Open the settings app on your Apple Watch. Scan down to Battery. If it's health is 80% or less, it's time.
Be advised that Apple doesn't actually replace the battery in Apple Watch. Instead, for a fee, they take your watch and give you a refurbished, but like new watch of the same model and color.
See this to get the process started if you need to schedule a battery service --> Apple Watch Service & Repair - Apple Support