How to update a custom post title from a front-end form using ACF fields?

You check if !$value before assigning the title from the fields, that test will be false once the post has a title. If you always want it to update, then remove the test.

function auto_title_insert() {
    return $_POST['fields']['field_538626f57e84c'].' '.$_POST['fields']['field_538627ffeccb0'].' '.$_POST['fields']['field_53863a5c7502b'].' '.$_POST['fields']['fields[field_53a9bb09f82ba]'];
}
add_filter( 'title_save_pre', 'auto_title_insert' );