title: pandas update dataframe row with new value
body: <p>I have a text file which has been read into pandas by <code>df1 = pandas.read_csv(r'fruits.txt', sep=',')</code></p>
  
  <pre><code>    item  freshness
  0   apple    2.2
  1   pear     0.0
  </code></pre>
  
  <p>and a series of calculations that would yield the result of <code>apple = 2.3</code></p>
  
  <p>Is it possible to do a <code>pandas.update</code> so that I can update the value of <code>freshness</code> for <code>apple</code> in the dataframe to <code>2.3</code>?</p>