how can i ristrict the user that they would only change those artifact in DNG which are linked in assigned WorkItem

Hi Gagan, when you say "User can only Modify that Linked Artifact not whole Document", with document you mean the module?

Anyway, DNG is a collaboration tool, and that implies that the tool is not that granular with its permissions to limit operations on specific artifacts for specific users.
You can assign permissions to specific roles for users, and create subteams with users, that can only edit artifacts in folders that are assigned to that subteam, but doing that with one or more artifacts in a module is not possible I think.