The theme credits are part of footer.php. To change them, place a copy of footer.php in your child theme folder and edit these lines as you like:
<a href="<?php echo esc_url( __( 'http://wordpress.org/', 'fictive' ) ); ?>"><?php printf( __( 'Proudly powered by %s', 'fictive' ), 'WordPress' ); ?></a>
<span class="sep"> | </span>
<?php printf( __( 'Theme: %1$s by %2$s.', 'fictive' ), 'Fictive', '<a href="http://automattic.com" rel="designer">Automattic</a>' ); ?>