My Apple Watch is losing charge what do I need to do ? My phone is updated

When you update your iOS to a new major release watchOS will also update. This will then necessitate a large number of app updates which will use more battery than you would normally use. This usually settles down after a couple of days

Force restart your Apple Watch

You should force restart your device as a last resort and only if it's not responding. To force restart your Apple Watch, press and hold both the side button and Digital Crown for at least 10 seconds, then release both buttons when you see the Apple logo.