In iOS 12 & up, swipe down from the top right corner, tap and hold the Music control window on the right and tap the AirPlay button to select the audio output source.
In iOS 11, bring up the Control Center by swiping up on the display, deep press on it (if you have a 3D Touch enabled iPhone) or press and hold the Music controls for a couple of seconds. Tap on the AirPlay button on the top right hand corner. (more info here)
In iOS 9.3.5, the AirPlay button is in the control center accessible by swiping up from the bottom of the screen.
In iOS 5 and 6 double click the home button, swipe the bottom row to the left and you’ll see the AirPlay button.
Please check out this link by Apple: