title: How to modify/update column values on a condition in Pydatatable?
body: <p>In pydatatable, I'm trying to modify a column values specifying a condition in i i.e <code>DT[i=="text", j="some"]</code></p>
  
  <p><strong>sample DT:</strong></p>
  
  <pre><code>py_DT= dt.Frame({'crossing':['ABC','A','B','B','A','A','ABC'],
                   'total' :[2,4,5,6,8,10,12]})
  </code></pre>
  
  <p>Here i would like to replace crossing value <code>'ABC'</code> with <code>'A'</code> only, for that i have written a below sample code,</p>
  
  <p><strong>Attempt 1:</strong></p>
  
  <pre><code>py_DT[f.crossing=="ABC", f.crossing=="A"]
  </code></pre>
  
  <p><strong>Attempt 2:</strong></p>
  
  <pre><code>py_DT[f.crossing=="ABC", update(f.crossing=="A")]
  </code></pre>
  
  <p>None of these attempts were worked out, is there any other way to get it solved? Could you please write to me how to update a column value as per the said requirement?</p>