i just found the answer - after a lot of digging:
i insert the code:
header("location: " . $_SERVER['REQUEST_URI']);
into update_slider_db function - after updating the db.
and then add:
//allow redirection, even if my theme starts to send output to the browser
add_action('init', 'do_output_buffer');
function do_output_buffer() {
ob_start();
}
into function.php