First:

Go to the root folder of Joomla >> languages >>en-GB/ en-GB.mod_footer.ini and modify it there.

FOOTER=Footer
FOOTER_LINE1=Copyright © %date% %sitename%. All Rights Reserved.
FOOTER_LINE2=<a href="http://www.joomla.org">Joomla!</a> is Free Software released under the <a href="http://www.gnu.org/licenses/gpl-2.0.html">GNU/GPL License.</a>
MOD_FOOTER=<em>mod_footer</em>
THIS MODULE SHOWS THE JOOMLA! COPYRIGHT INFORMATION=This Module shows the Joomla! Copyright information

Second:

Go to yourtemplate/index.php >> bottom of page and change it there.

<p id="power_by">
<?php echo JText::_('Powered by') ?> <a href="http://www.joomla.org">Joomla!</a>.
<?php echo JText::_('Valid') ?> <a href="http://validator.w3.org/check/referer">XHTML</a> <?php echo JText::_('and') ?> <a href="http://jigsaw.w3.org/css-validator/check/referer">CSS</a>
</p>


After that you will have your own custimzed copyright information.