How to pairing 1st generation iWatch with iphone6 (ios11.2.6)?

I have a 1st generation iWatch which has been used before. Today when i am trying to pairing it with iPhone6, initially pairing is ok, but it continuously failed when upgrading iWatch OS. iphone keeping claiming iWatch OS upgrade failure due to network issue, but my iphone wifi connection is perfect and other applications are ok. Looks to me like a IOS bug, can anyone suggest how to resolve/workaround this issue? Thanks!