How do I update my Apple Watch manually?

See -> Update your Apple Watch

Update your Apple Watch – Apple Support (UK)

• Leave your Apple Watch on its charger while it's updating. Don't restart your iPhone or Apple Watch, and don't close the Apple Watch app. When the update has completed, your Apple Watch will restart on its own.

See also the following Apple articles:

The Apple Watch app -> The Apple Watch app – Apple Support (UK)

Apple Watch user guide -> https://books.apple.com/book/id6442854652

If you want to update manually, rather than automatically, go to your Watch Settings > Software Updates. Click on Automatic Updates and toggle off.