Assuming you have created a child theme inside your main wordpress templates. If not, Here’s How to create a child theme.
1. Copy footer.php from your main templates to your child-theme folder.
2. Edit footer.php and find the following code:
<?php do_action( 'twentythirteen_credits' ); ?>
<a href="<?php echo esc_url( __( 'http://wordpress.org/', 'twentythirteen' ) ); ?>" title="<?php esc_attr_e( 'Semantic Personal Publishing Platform', 'twentythirteen' ); ?>"><?php printf( __( 'Proudly powered by %s', 'twentythirteen' ), 'WordPress' ); ?></a>3. Modify the code to:
<?php do_action( 'twentythirteen_credits' ); ?> <a href="YOURSITE.COM" target="_blank" title="TITLE OF THE LINK"><?php printf( __( 'YOUR COPYRIGHT TEXT', 'twentythirteen' ), '' ); ?></a>
4. Save & Refresh your website. (If not showing clear your browser cache first)