title: Alter Datetime format in Python
body: <p>I'm trying to modify date format in date column of a python dataframe 'df' from 06/01/2018 to 06-01-2018.</p>
  
  <p>I tried the following but did not work.</p>
  
  <pre><code>df['date'].str.replace('/','-')
  </code></pre>
  
  <p>Any help is really appreciated.</p>