The Joomla copyright text defined in language file: /language/en-GB/en-GB.mod_footer.ini
You can translate or change this line:
MOD_FOOTER_LINE2="<a href="_QQ_"https://www.joomla.org"_QQ_">Joomla!</a> is Free Software released under the <a href="_QQ_"https://www.gnu.org/licenses/gpl-2.0.html"_QQ_">GNU General Public License.</a>"
In case that you want to remove both completely, go to file: /templates/ja_mixstore/html/mod_footer/default.php
and change:
<div class="module">
<small><?php echo $lineone; ?> Designed by <a href="http://www.joomlart.com/" title="Visit Joomlart.com!" <?php echo method_exists('T3', 'isHome') && T3::isHome() ? '' : 'rel="nofollow"' ?>>JoomlArt.com</a>.</small>
<small><?php echo JText::_( 'MOD_FOOTER_LINE2' ); ?></small>
</div>
to:
<div class="module">
<small><?php echo $lineone; ?></small>
</div>