How to sync data using Core Data in WatchOS 2?

If you are talking about syncing it between the app and watchOS 2, I think Watch Connectivity Framework is the only ways (as far as I can think of) to communicate between the app and watchOS 2. So you will need to establish a coommunication scheme to achieve that.

I havn't tested on watchOS 2, but I hope and assume this didnt change. What I do is store my core data model in a shared framework between the watchOS and the phone. If you have an existing app, I dont know how hard it will be to move it into the shared folder but I am sure its possible.