How to temporarily redirect front page of a network to one of the sites/blogs?

This code goes in your child themes functions.php file.

add_action( 'template_redirect', 'wpsites_redirect_ms_front_page' );

function wpsites_redirect_ms_front_page() {
    if ( is_main_site() && is_front_page() ) {
        wp_redirect( 'http://www.example.dev/', 302 );
        exit;
    }
}

Something like this should work (untested).

Modify Code

Simply swap out the URL in the code above.

You can also change the 302 to 301 or any other type of redirect.