title: Finding an element in nested python list and then replacing it
body: <p>I have a nested list and I am trying to replace a certain element of the list with something else.</p>
  <pre><code>  NL = [[1,2,3],
         [4,5,6],
         [7,8,9]];
  </code></pre>
  <p>Now, I need to update the list, let's say the user wants to change element at NL[1][1] (i.e. 5) to 'X'.</p>
  <p>NL will be updated as</p>
  <pre><code>  NL = [[1,2,3],
         [4,'X',6],
         [7,8,9]];`
  </code></pre>
  <p>I am having trouble trying to find the position of the element and then changing it. Any help is much appreciated.</p>
  <p>Thanks</p>