You can change it on this file
<blockquote>Open templates/ja_t3v3_blank/tpls/blocks/footer.php file</blockquote>
<?php
$t3_logo = $this->getParam ('setting_t3logo', 't3-logo-light', 't3-logo-dark');
if ($t3_logo != 'none') : ?>
<div class="poweredby">
<small><a href="http://t3.joomlart.com" title="Powered By T3 Framework" target="_blank">Powered by <strong>T3 Framework</strong></a></small>
</div>
<?php endif; ?>
And ini text file
<blockquote>language/en-GB/en-GB.mod_footer.ini</blockquote>
from
MOD_FOOTER_LINE2="<a href="_QQ_"http://www.joomla.org"_QQ_">Joomla!</a> is Free Software released under the <a href="_QQ_"http://www.gnu.org/licenses/gpl-2.0.html"_QQ_">GNU General Public License.</a>"
change to
MOD_FOOTER_LINE2=""