iPhone rotation locking has been moved to the Control Center. To enable rotation lock and prevent the screen from automatically changing orientation when the iPhone is turned:
To access Control Center and lock rotation, follow these steps:
1. Swipe up from the bottom of the home screen to bring up Control Center.
2. The top row of the Control Center shows 5 circular toggles.
The furthest right icon looks like an arrow around a padlock - touch this icon to enable or disable rotation lock on your device.
3. Swipe down to dismiss the Control Center.