Apple Watch Series 0 keep rebooting after installed watchOS 4.2

Hi

It is certainly not normal for your Apple Watch 1st generation (there is no "Series 0" under Apple's terminology) to restart unexpectedly.

Whilst I can't say why that has been happening, the following steps may help to stop if from happening again (check performance after each before moving on to the next):

Close all apps and restart both your iPhone and your Apple Watch (normally):

• Close all open apps on your iPhone.

Unpair and re-pair your watch:

• The Watch app on your iPhone takes a backup of your watch automatically when unpairing via the app.
• Initially choose to Restore from Backup during set up (most data and settings will be restored, with some exceptions, such as Apple Pay cards, passcode).
• If the issue persists, consider instead unpairing and setting it up as a new watch.
• Unpair your Apple Watch and iPhone - Apple Support
• Set up your Apple Watch - Apple Support

Remove apps and watch face complications:

• Remove third-party apps from the Dock and/or remove them completely from your watch and then monitor performance after adding them back, one at a time.
  • Open apps
• Remove watch face complications and monitor performance as you add them back:
  • Change the watch face on your Apple Watch - Apple Support

If the issue then persists, I suggest that you contact Apple Support (remote diagnostics and/or mail-in service may be available, if required), make a Genius Bar reservation or visit an Apple Authorised Service Provider for assistance:

• US: Apple Watch Service Answer Center - Apple Support
• Elsewhere: Select your country for Apple Watch service and repair information

You're welcome!

Follow the instructions here to unpair AirPods from your Apple Watch:

Use Bluetooth accessories with Apple Watch - Apple Support

Alternatively, you may wish to unpair them from your iPhone and set them up again:

Use your AirPods - Apple Support