Let's assume that you have started a simulation locally on your machine. First of all, select the Servers window of the Explorer pane and see your simulation listed there.
Reselect the simulation window in the Explorer pane, and select the File > Disconnect menu item:
This will disconnect the client from the server, but both will still be running, although you will no longer see the simulation window in the client.
To close the client and leave the server running, select the File > Exit menu item, but you could leave the client running (to open another simulation, for example).
To shut down the server as well as the client, simply use the File > Exit menu item while the server is still connected to the client.