You need to edit file function.php inside plugin folder; at line 181 you can find the code creating that string.
Note that DateTime class i’m using requires PHP 5.2.0 or later.
Just change this:
$event .= ' <div class="snippet-data-img"> <span itemprop="startDate" datetime="'.$event_start_date.'T00:00-00:00">'.$event_start_date.'</span></div><div class="snippet-clear"></div>';
into this:
$myStartDate = DateTime::createFromFormat('m/d/Y', $event_start_date);
$my_event_start_date = $myStartDate->format('d/m/Y');
$event .= ' <div class="snippet-data-img"> <span itemprop="startDate" datetime="'.$event_start_date.'T00:00-00:00">'.$my_event_start_date.'</span></div><div class="snippet-clear"></div>';
You can change EndDate too editing relative variables.