Switch to External Audio

Renting a house with RCA 40" LED40G45RQ TV. I am trying to hook up an external home theater audio. I cant find any command which allows me to switch audio to external source. Of course, the previous tenant lost remote but I still can access and move among menu and commands....I just dont see anything which enables me to switch. The red white cables are hooked up properly, in fact I get feedback when I adjust them. On other TVs, its a simple command.....

Please help ??