The screen orientation can be locked on any device running iOS 4 or later, except for the 2nd generation iPod touch.
To lock the screen orientation in portrait mode on an iPhone or iPod touch, the first step is to double tap on the home button . This displays a list of recently opened apps. However, there is a screen to the left of this menu, which should display music controls, and, on the left, orientation lock. You can access this screen by swiping your finger to the right.

 The leftmost icon on this screen toggles rotation lock

 This indicates that the orientation is unlocked. Tap it to lock in portrait orientation.

 This indicates that the orientation is locked. Tap it to unlock the orientation.

Swipe from bottom to top on the screen to reveal Control Center (if you do not see it, go to Settings > Control Center and make sure it is on). Of the five buttons displayed at the top, the rightmost one will toggle portrait orientation lock.
    
Swipe up to reveal Control Center, then tap the lock with a circle around it in the upper right of Control Center

When the icon is white, orientation lock is on.


When the icon is black, orientation lock is off.


You can swipe downward or press the home button to exit Control Center.