So you want to keep your footer module and just delete or change the ”Joomla! is Free Software released under the GNU/GPL License” from Joomla1.5
In the root directory go to
> modules\mod_footer\tmpl\default.php
In this line either comment it or delete it
<div><?php echo JText::_( ‘FOOTER_LINE2′ ); ?></div>
Save and replace.