function change_update_cart_text( $translated, $text, $domain ) {
    if( is_cart() && $translated == 'Update cart' ){
        $translated = 'Sepeti Güncelle';
    }
    return $translated;
}
add_filter( 'gettext', 'change_update_cart_text', 20, 3 );

function change_update_apply_coupon( $translated, $text, $domain ) {
    if( is_cart() && $translated == 'Apply coupon' ){
        $translated = 'Kuponu Uygula';
    }
    return $translated;
}
add_filter( 'gettext', 'change_update_apply_coupon', 20, 3 );

function change_update_coupon_code_text( $translated, $text, $domain ) {
    if( is_cart() && $translated == 'Coupon code' ){
        $translated = 'Kupon Kodu';
    }
    return $translated;
}
add_filter( 'gettext', 'change_update_coupon_code_text', 20, 3 );