Showing the date of the latest edit, instead of node's posting date I have some nodes I update frequently, but in content pane I can see the posting date of node.  

How can I make Drupal 7 show the date of the latest edit, instead of the posting date?
I would tackle this in a template_preprocess_node, either in a module or in your theme.

function mytheme_preprocess_node (&amp;$variables)
{
   $variables['date'] = format_date($variables['node']-&gt;changed);
}


Depending on your node template, you may also want to do something like

function mytheme_preprocess_node (&amp;$variables)
{
   $variables['date'] = format_date($variables['node']-&gt;changed);

   if (variable_get('node_submitted_' . $variables['node']-&gt;type, TRUE)) {
     $variables['display_submitted'] = TRUE;
     $variables['submitted'] = t('Edited by !username on !datetime', array('!username' =&gt; $variables['name'], '!datetime' =&gt; $variables['date']));
     $variables['user_picture'] = theme_get_setting('toggle_node_user_picture') ? theme('user_picture', array('account' =&gt; $node)) : '';
   } else {
     $variables['display_submitted'] = FALSE;
     $variables['submitted'] = '';
     $variables['user_picture'] = '';
   }    
}


so that your new $date gets used in $submitted, due to the preproccess/process order.

You can also play around with variants.  For example, you could have both created and last edited dates; you would juust need to set up the variables in the preprocess function, and then use them in your node.tpl.php.