Change the WP Logo Link to your site and the text in the tooltip.

functions_wplogin.php

function myprefix_loginpage_custom_link() {
    return get_bloginfo( 'url' );
}
add_filter('login_headerurl','myprefix_loginpage_custom_link');

function myprefix_change_title_on_logo() {
    return 'My Tooltip';
}
add_filter('login_headertitle', 'myprefix_change_title_on_logo');