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