Hi,
click on appearance -> editor then open footer.php
here you can edit the text/link in the footer area.
<footer id="colophon" role="contentinfo">
<div class="site-info">
<?php do_action( 'twentytwelve_credits' ); ?>
" title="<?php esc_attr_e( 'Semantic Personal Publishing Platform', 'twentytwelve' ); ?>"><?php printf( __( 'Proudly powered by %s', 'twentytwelve' ), 'WordPress' ); ?>
</div><!-- .site-info -->
</footer>