manage Offline+Online Database for winform application

Synching data between relational databases is a pain. Your best course of action is probably dependent on: how many users will there be? How probably are conflicts (i.e. that the users will work offline on the same data). Also possibly what kind of manpower do you have (do you have proper DBAs/Sql Server devs standing by to assist with the SQL part, or are you just .NET devs)

Prepare database on client pc (Use same structure as on server) and use that database only for transactions with application. and sync that offline data to online data on time interval. For that you can create and windows service or sql jobs or even yo can use sql sync algorithm which executes on time interval.