First, Double click the home button on the device--it doesn't matter if Pandora is the active window, as long as it's playing.

Swipe to the right along the area at the bottom to reveal the music player controls and Pandora icon. Tap the AirPlay button (to the right of the forward button) and select the other source you want to stream to.

The AirPlay button only shows when another source (Airport Express/Apple TV) is available on that network.