title: How to Replace some value of First Row as "-" in R
body: <p>I have below mentioned dataframe:</p>
  
  <pre><code>Month   ID   V1   V2    V3     V4     V5     V6    V7
  Mar-17  1    2    3.5%  4      6      8      1.5%  15
  Feb-17  1    2    2.5%  2      3      5      5.5%  15
  Jan-17  1    2    7.5%  1      8      1      3.5%  15
  </code></pre>
  
  <p>I want to replace <code>V2</code>, <code>V4</code> and <code>V6</code> as "-" of the dataframe for the first row only.</p>