Watch battery

Get an estimate using this support page:--> Apple Watch Service & Repair

Then make an appointment (*) and have your device serviced at an Apple Store or AASP (Apple Authorized Service Provider) among this list:--> https://locate.apple.com/choose-country-region .

Before having it serviced, perform these steps:--> Get your Apple Watch ready for service

Regards

Giulio

(*) You can do it:

• Online:--> Genius Bar Reservation and Apple Support Options - Apple;
• Contacting a local Apple Store or AASP among the list provided above;
• Using the "Apple Store" app, available and downloadable from AppStore.