How to change column titles in the discussion_topics shortcode

You can change column titles from the default like this:

function myprefix_change_ctdb_titles( $titles ) {
  $titles['started'] = 'Posted';
  return $titles;
}
add_filter( 'ctdb_topic_titles', 'myprefix_change_ctdb_titles' );

The default titles are:

$titles = array(
 'avatar' => '',
 'topic' => __( 'Topic', 'discussion-board' ),
 'replies' => __( 'Replies', 'discussion-board' ),
 'started' => __( 'Since', 'discussion-board' ),
 'posted-by' => __( 'Posted by: ', 'discussion-board' ),
);

Just replace with new values as required.