title: Change value according to match in another tab in pandas
body: <p>Hello I have two df such as</p>
  <p><strong>df1</strong></p>
  <pre><code>COL1 COL2
  A 1
  A 2
  B 3
  B 4
  C 5
  D 6
  E 7
  F 8 
  </code></pre>
  <p><strong>df2</strong></p>
  <pre><code>Old New 
  A  A.1
  C  C.1
  E  E.1 
  </code></pre>
  <p>and I would like to replace <code>COL1</code>values matching in <code>df2.Ol</code>d by the <code>df2.New</code> value.</p>
  <p>And get :
  <strong>df1</strong></p>
  <pre><code>COL1 COL2
  A.1 1
  A.1 2
  B 3
  B 4
  C.1 5
  D 6
  E.1 7
  F 8 
  </code></pre>
  <p>Does someone have an idea please ?</p>