I replied you in the ticket already. I will re-post it here:
That text belongs to the footer module, you can change in the language file:
MOD_FOOTER_LINE1="Copyright © %date% %sitename%. All Rights Reserved." ; Note : %date% will be auto replaced by current year! Don't translate
MOD_FOOTER_LINE2="<a href=\"https://www.joomla.org\">Joomla!</a> is Free Software released under the <a href=\"https://www.gnu.org/licenses/gpl-2.0.html\">GNU General Public License.</a>"
You can go to Backend >> Manage >> Language Overrides and add override for above language constant: MOD_FOOTER_LINE2