title: How to replace part of dataframe in pandas
body: <p>I have sample dataframe like this</p>
  
  <pre><code>df1=
  
  A B C
  a 1 2
  b 3 4
  b 5 6 
  c 7 8
  d 9 10
  </code></pre>
  
  <p>I would like to replace a part of this dataframe (col A=a and b) with this dataframe</p>
  
  <pre><code>df2=
  
  A B C
  b 9 10
  b 11 12
  c 13 14
  </code></pre>
  
  <p>I would like to get result below</p>
  
  <pre><code>df3=
  
  A B C
  a 1 2
  b 9 10
  b 11 12
  c 13 14
  d 9 10
  </code></pre>
  
  <p>I tried</p>
  
  <pre><code>df1[df1.A.isin("bc")]...
  </code></pre>
  
  <p>But I couldnt figure out how to replace.
  someone tell how to replace dataframe.</p>