Are you unable to update to WatchOS 6.1.x? continue reading for a fix

0) charge your iPhone and Watch above 50%

1) unpair your watch using the Watch app

2) delete the Watch app from your phone

3) reset your Watch to factory default > press and hold the buttons at both sides until it restarts

3) reset the Network settings on your iphone General / Reset / Reset Network Settings – it will restart now

4) join your WiFi network again after the restart

5) download the Watch app from AppStore

6) pair your Watch with iPhone

7) update to OS – you should see 6.1.2, but might see 6.1.1 first and as a next step be offered with 6.1.2

you need iOS 13 on your iPhone in order to be able to update your Watch to 6.x.x