title: How to modify the constraint created by constraintsWithVisualFormat?
body: <p>for example : I had create constraints using constraintsWithVisualFormat</p>
  
  <pre><code>[v1 addConstraints:
       [NSLayoutConstraint
        constraintsWithVisualFormat:@"V:|[v2(10)]|"
        options:0 metrics:nil views:vs]];
  </code></pre>
  
  <p>but when I decide to change V1's height to V:|[v2(10)]-100-|,because constraintsWithVisualFormat  returns a array of constraints , and I don't know which constraint of the array is the one I have to modify ,how should i do this ?not using constraintsWithVisualFormat?</p>