title: how to rename a column in linux?
body: <p>I have a table like this </p>
  
  <pre><code> bedGraph  chrom=chr1 chr1:0-000
      1000    0
      2000    0
      3000    0.003802
      4000    0.046175
   bedGraph  chrom=chr2 chr2:5000-6000
      5000    0.103756
      6000    0.161337
  
   EXPECTED RESULTS
  
  bedGraph  chrom=chr1 rep
      1000    0
      2000    0
      3000    0.003802
      4000    0.046175
   bedGraph  chrom=chr2 rep
      5000    0.103756
      6000    0.161337
  </code></pre>
  
  <p>and I would like to rename the 3rd col only (the 2nd cols changes along the file). Is possible to do that using <code>sed</code>? 
  Thanks!</p>