Hello,
For that you need to first copy “footer.php” file from “Flat” theme and paste it in “Flat-Sky” theme directory. Now edit that file and add the following code
<a href="#">Your Copyright text</a>
In place of
<a href="<?php echo esc_url( __( 'https://www.ads-software.com/', 'flat' ) ); ?>" title="<?php esc_attr_e( 'Semantic Personal Publishing Platform', 'flat' ); ?>"><?php printf( __( 'Proudly powered by %s', 'flat' ), 'WordPress' ); ?></a>.
<?php printf( __( 'Theme: %1$s %2$s by %3$s.', 'flat' ), 'Flat', wp_get_theme()->get( 'Version' ), '<a rel="nofollow" href="'.esc_url( 'https://themeisle.com/themes/flat/' ).'" title="'.esc_attr( 'Flat WordPress Theme' ).'">Themeisle</a>' ); ?>
In place of “#” you can put your link address, and in place of “Your Copyright Text” you can add your text.
Thank You