title: update data frame variable according to another table
body: <p>I have two <strong>data frames</strong> Table 1 and Table 2 each one with two variables (V1, V2)
  What I want to do is to update the V2 for each row of Table 1 according to V2 of Table 2.</p>
  
  <p>For example, in Table 1 there are two rows with V1==27 and their respectively values of V2 are 6 and 7. According to Table 2 every V1==27 should be 6.5 so I want to update every V1==27 from Table 1 and set its V2=6.5.</p>
  
  <p>How can I accomplish that with R?</p>
  
  <pre><code>Table 1
    V1     V2
     1     1
     5     2
     18    3
     24    4
     25    5
     27    6
     27    7
  
  Table 2
    V1     V2
     1     1.0
     5     20000.0
     18    3.0
     24    4.0
     25    5.0
     27    6.5
  
  Resulting Table 3
    V1     V2
     1     1.0
     5     20000.0
     18    3.0
     24    4.0
     25    5.0
     27    6.5
     27    6.5
  </code></pre>