title: Pandas- rename dataframe multilevel header according to the name of the first level header
body: <p>I have a dataframe like this :</p>
  
  <pre><code>    X         Y
      a    b    a    b
  0   1    3    4    2
  1   5    7    8    6
  </code></pre>
  
  <p>And I want to rename a specific column name, for example "b" to "b1" under "Y" header. The desired result is</p>
  
  <pre><code>    X         Y
      a    b    a    b1
  0   1    3    4    2
  1   5    7    8    6
  </code></pre>
  
  <p>So it is important, that header "b" under "X" remained unchanged. That mean i can't just use <code>rename</code></p>