title: DynamodDB: How to update sort key?
body: <p>The table has two keys: <code>filename</code> (partition key) and <code>eventTime</code> (sort key). 
  I want to update <code>eventTime</code> for certain <code>filename</code>. Tried <code>put_item()</code> and <code>update_item()</code> sending the same <code>filename</code> with new <code>eventTime</code> but those functions add a new item instead of update. </p>
  
  <p>What should I use for that purpose?</p>