Just download the child theme child-theme-105.zip shared in this reply https://colorlibsupport.com/t/modify-footer-for-mobile-version/#post-53854
That custom child theme of Activello theme that contains following custom code in the functions.php file so just edit the text in this file to whatever you want to.
function custom_activello_footer_info() {
global $activello_footer_info;
printf( esc_html__( 'Theme by %1$s Powered by %2$s', 'activello' ) , '<a href="https://colorlib.com/" target="_blank">Colorlib</a>', '<a href="http://wordpress.org/" target="_blank">WordPress</a>');
}