title: Change dictionary key by value
body: <p>How to change key by value in <code>Dictionary</code>?</p>
  
  <p>I have code like this:</p>
  
  <pre><code>public Dictionary&lt;string, int&gt; Groups = new Dictionary&lt;string, int&gt;();
  Groups.Add("apple", 0);
  </code></pre>
  
  <p>And I need do like this:</p>
  
  <pre><code>Groups.Update("0") = "pear"
  </code></pre>