Mac/Snow Leopard: Upper right hand corner of screen - Click on the Network icon, Turn Airport on/off
Windows/Vista: Lower right hand corner of screen - Click on the Network icon, Disconnect/connect
These instructions assume (usually means big problems) that WiFi is being used and that the Icons are set to be displayed (fairly normal).
If these directions don't fit your situation, please dig around on your own because...
On Windows, in particular, there are lots of different paths to the same thing and you can create shortcuts to the objective, e.g., you can go to Start (lower left hand corner of screen), Choose Control Panel. Choose Connect to a network (under Network and Sharing Center), and then Disconnect/connect.