title: ios change constraint programmatically
body: <p>I am using autolayout with a constraint.
  The top space to superview is 100</p>
  
  <p>I now want to change this so that when the user rotates the device to landscape
  The top space to superview is 50</p>
  
  <p>Can this be done at design time?</p>
  
  <p>How can this be done at runtime?</p>