Some Apple Watch Series 4 users have reported that updating their wearable to watchOS 5.1 bricked their device. After rebooting the Watch when installing the update, the device would freeze on the Apple logo and become unresponsive. Apple has since pulled the update and is working on a fix. In the meantime, affected users have been instructed to contact AppleCare for assistance.
https://www.notebookcheck.net/Updating-to-watchOS-5-1-bricks-some-Apple-Watches-Apple-working-on-fix.354896.0.html
it's not a problem with the update! it's a new feature from apple: survival of the fittest watch ;D ;D ;D ;D