title: Unix code wanted to change column values
body: <p>I have a file that looks like this:</p>
  
  <pre><code>ID1 ID2 3
  ID2 ID3 3
  ID4 ID5 3
  ID6 ID7 4
  ID8 ID9 4
  ID8 ID6 4
  </code></pre>
  
  <p>And I want to change it to:</p>
  
  <pre><code>ID1 ID2 1
  ID2 ID3 1
  ID4 ID5 1
  ID6 ID7 2
  ID8 ID9 2
  ID8 ID6 2
  </code></pre>
  
  <p>So I want to change all 3's and 4's from the third column to 1's and 2's respectively. What is the most efficient way to do this for a very large file? Many thanks in advance!</p>