title: Replace values in pandas dataframe based on other dataframe
body: <p>I have two dataframes, one in the form of:</p>
  
  <pre><code>#   X   Y   
  1   2   0.0 
  2   5   0.0 
  3   10  0.0 
  4   15  0.0 
  5   17  0.0 
  6   21  0.0 
  </code></pre>
  
  <p>and one in the form of:</p>
  
  <pre><code>A   B   C   
  1   4   2   
  2   5   3   
  3   6   4   
  </code></pre>
  
  <p>I want to replace all the ABC values from the second dataframe, with the X values; so I want go over the ABC df and if the number matches the # of df1 to replace it with the X value</p>
  
  <p>the end table should look:</p>
  
  <pre><code>A   B   C   
  2   15  5   
  5   17  10  
  10  21  15   
  </code></pre>
  
  <p>is there a way I can do it?</p>