title: Access only second row of column names in a dataframe
body: <p>I want to read an excel file where the second line is a date in a string format and the first line is the weekday that corresponds to each date, and then change the second line from string to datetime. If I only read the second line as index, and completely skip the first line with the days, I do the following to convert it to a datetime:</p>
  
  <pre><code>Receipts_tbl.columns = pd.to_datetime(Receipts_tbl.columns)
  </code></pre>
  
  <p>How do I do that if I have a multiindexed dataframe, where the first line of the indices remains as weekdays, and I want the second to be converted to datetime? </p>
  
  <p>Thanx</p>