This may not be proper coding, but this solution worked for me. Change this code in footer.php
<?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>
to this code (replace with your site info)
<?php do_action( 'twentythirteen_credits' ); ?>
<a href="YOUR SITE URL HERE" title="YOUR SITE TITLE HERE"><?php printf( __( 'YOUR COPYRIGHT INFORMATION HERE', 'twentythirteen' ), '' ); ?></a>