How can i sync offline database and online database

Fairly a complicated job. You need to look into primary keys, foreign keys, all data modifications such as insert / update and delete to keep in sync. If one db is changing and you want to keep another in sync, that is fine. However, two way sync is pretty much a project by itself.

Technically, this is not a C# work, try to see if scripts in native db would do the job. C# would come into picture if you want to call scripts at click of a button.