How do you send data from the client to the server?

It’s not exactly clear what you want. Do you want to send things via RPC to the server? That’s in the tutorial.

For replicated variables, the server has authority. So for the client to dictate anything you’re pretty much limited (rightfully so) to RPC.

You really need to be more clear what you’re trying to accomplish with this client-server relationship. The answer to this question is dependent on this.