Change link for “add new” buttons on custom post type

in most of the case WordPress ui uses admin_url() function to retrieve administrative urls. so you can use admin_url filter to modify that.

Example:

add_filter( 'admin_url', 'wpse_271288_change_add_new_link_for_post_type', 10, 2 );
function wpse_271288_change_add_new_link_for_post_type( $url, $path ){
    if( $path === 'post-new.php?post_type=your_custom_post_type' ) {
        $url = get_permalink(/*your post ID here*/); // or any other url
    }
    return $url;
}

Change your_custom_post_type with your custom post type name.

Reference (WordPress Version 4.8): edit.php (Line No. 312) and link-template.php (Line No. 3132)