title: Python / Pandas Dataframe change specific value
body: <p>I have a column in DataFrame a <code>Start_Time (datatype: Object)</code> with values as :-</p>
  
  <pre><code>6:00:00
  7:00:00
  8:01:00 
  </code></pre>
  
  <p>and want to change it to </p>
  
  <pre><code>6:30:00
  7:30:00
  8:30:00
  </code></pre>