\documentclass[a4paper,11pt,fleqn]{book}
\usepackage[polish]{babel}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{graphicx}
\usepackage{newtx}
\usepackage{titlesec}
\usepackage{pifont}
\usepackage{geometry}

\geometry{width=126mm}

\titleformat{\chapter}[display]{\LARGE\bfseries}
{\textsc{\Huge Rozdział} \thechapter}{10mm}{\ding{103}~~}

%\titleformat{\chapter}[frame]{\huge\bfseries}
%{\textsc{Rozdział} \thechapter}{7mm}{\ding{103}~~}

%\titleformat{\chapter}[hang]{\huge\bfseries}
%{\Huge \thechapter}{10mm}{\ding{103}~~}

\titlespacing{\chapter}{0pt}{18mm}{18mm}

\titleformat{\section}[display]
{\filcenter\Large\bfseries}{\filcenter\thesection}{0mm}{}
\titlespacing{\section}{0mm}
{8mm plus 0mm minus 1mm}{4mm plus 0mm minus 1mm}


\begin{document}

\chapter{Wprowadzenie}
\label{cha:wprowadzenie}


\chapter{Podstawy modelowania}
\label{cha:podstawy:modelowania}

\section{Proces modelowania i weryfikacji systemów}
\label{sec:proces:modelowania}

\end{document}