\documentclass{book}
 \usepackage{xepersian}
 \settextfont{XB Zar}
 \setlatintextfont{Junicode}
 \setdigitfont{XB Zar}
 \begin{document}
\renewcommand\thechapter{م} 
\chapter{اول}
\section{بخش اول}
\section{بخش دوم}
\setcounter{chapter}{0}
\renewcommand\thechapter{\arabic{chapter}} 
\chapter{دوم}
\section{بخش اول}
 \end{document}