How does server call clinet methods?

I want to pass data between server and client. In RMI, client can call remote server methods, but how can server call client methods?

By reversing the normal process of passing a stub.
A server isn't so much a machine or a process -- it's a role that a program can play in a distributed environment. In this sense with RMI, a server is any process that exports a stub that can be passed to a client for use. A client is any process that is "aware" of the service interface and will use the stub to call on a server's methods.
A typical scenario might look like this:
1) One client process calls a server process and gets a stub.
2) The "initial client" passes a stub to the "initial server" as a parameter via one of the server's exported methods.
3) The initial server calls on the initial client via one of its exported methods, passing data as a parameter.
High-level view, but I hope it gives you the gist.