How do I unlock portrait mode in iPhone?

My screen has a lock with an almost full bordering circle around it ending with an arrow. This is located in the top right of my screen. Just inside the battery %. How do I turn this lock off so I can use landscape mode?

Solution 1:

This is the lock rotation icon. You will notice that when you turn your iPhone (say, in Safari) it will stay in portrait.

To disable it:

On iOS 7+ :

• Open the Control Centre by sliding your finger from the bottom of the screen to the top
• Tap the button a the top right (here circled in red)
  

iOS 6

• Double tap on home button
• Then slide the multitask bar to the left (where the Sound controls are)
• And on the left you'll find the button to unlock this.

Solution 2:

On iOS 7+:

• Slide your finger from the bottom of the screen upwards. A menu called Control Center should appear.

• Click on the lock icon.