Are you talking about the title of the page that shows up in the title bar of a web browser as shown in this picture?
You can edit that in the .php files. Every php file should have code that looks like this that sets the page title
That code is in profile.php and sets the page title for the profile page.
$main_smarty->assign('posttitle', " / " . $main_smarty->get_config_vars('PLIGG_Visual_Profile_ModifyProfile'));
In /templates/your_template/pligg.tpl you will see where <title></title> is used in the html code