title: How to find-and-replace specific string in a file?
body: <p>I have data like this:</p>
  
  <pre><code>Ind M1  M2  M3  M4  M5
  P1  A/A Unused  G/A T/T T/T
  P2  T/T A/A A/A A/A G/G
  1   T/A A/A G/A T/T G/G
  2   Unused1 A/A G/A T/T T/G
  3   T/T A/A G/A T/T T/G
  4   Unused1 A/A G/A A/T G/G
  5   T/A A/A G/A A/T Unused1
  </code></pre>
  
  <p>and I want to replace <code>T/T</code> with <code>TT</code> in a whole file. 
  Please find link to my data here</p>
  
  <p><a href="https://www.dropbox.com/s/r1eib3grr11tjpw/data.txt?dl=0" rel="nofollow noreferrer">https://www.dropbox.com/s/r1eib3grr11tjpw/data.txt?dl=0</a></p>
  
  <p>I used gsub function, but what I am getting is strange output, like
   <code>"c(6, 7, 1, 2, 3, 4, 5)" "c(1, 3, 2, 4, 3, 4, 2)" "c(2, 1, 1, 1, 1, 1, 1)</code></p>
  
  <p>How to replace this <code>T/T</code> with <code>TT</code>?</p>