I tried every other way mentioned in the blogs and post to update my apple watch 3 via iphone 12, but didnt worked. Everytime it prompted no internet, try again even though internet was flawless. I tried reseting network settings, hard reset etc and etc, but still back to square one. What clicked for me was quite very easy and is as below. Solution : Reset your apple watch : Press crown and power button simultaneously until it switches off. If the apple watch was already paired , in the bluetooth section of your iphone , click on forget the device. Now , if your iphone is connected to the wifi network on 5G or if your phone has internet coming in as 5G , you need to change that to lower version by selecting a lower version wifi signal in your iphone. Reason , apple watch 3 doesnt support 5G and pairing/updating it over the same network will always prompt no internet. Now after selecting the lower wifi version in your iphone, connect to it. Switch on and pair your apple watch with your iphone. It will start the update post clicking on update now in apple watch app on the iphone. Thanks