Your account must be promoted from "authenticated user" to "cmrn member" by any of the existing cmrn members (e-mail cmrn@criticalmassradioSTOPSPAM.net for help).
Take a look at the "Stream switcher" block on the right sidebar.
The station which is currently streaming should be listed as "Now relaying".
Click on the dropdown menu and select the station you want to switch to.
If the station you want to switch to isn't listed, then it is either offline, or it has not been added to the icecast stream switcher. In the former case, contact the station; in the latter case, e-mail cmrn@criticalmassradioSTOPSPAM.net for help.
Click "switch stream".
The switch should happen immediately, though there may be a delay in when you hear it due to buffering by your audio player.