I found this solution:
1.Log in to your dashboard and go to Appearance > Editor.
2.Select the footer.php file from Templates column.
3.Now find the following piece of code between <div id=”site-generator”> and <div id=”site-generator”> tags.
<?php do_action( 'twentyeleven_credits' ); ?>
<a>" title="<?php esc_attr_e( 'Semantic Personal Publishing Platform', 'twentyeleven' ); ?>" rel="generator">
<?php printf( __( 'Proudly powered by %s', 'twentyeleven' ), 'WordPress' ); ?></a>
4.Remove the code, press Update File button to save the changes.
Once the code is removed, you can place your links, copyright message and other information between the tags.
NOTE: If you are using WordPress MultiSite, then the changes are made under Network Admin>Themes>Edit.