Older iWatch Update Issue

I received a new iWatch for Christmas and gave my old one to my mother. I am trying to pair my former iWatch to her iPhone 8 but have gotten stuck at the watch software update phase. The IOS is up to date and I have deleted the iWatch from my phone. I’m able to start the iWatch software update process but it never goes beyond the message that it is updating and may take awhile. We waited for an hour on one attempt before turning the phone completely off and restarting. Can someone please help or point me in the right direction in the manual on what to do?