\documentclass{book}
\usepackage{amssymb,amsmath}
\usepackage{empheq}
\usepackage{xepersian}

\makeatletter
\@ifundefined{Umathcode}{\let\Umathcode\XeTeXmathcode}{}
\@ifundefined{Umathchardef}{\let\Umathchardef\XeTeXmathchardef}{}
\makeatother
\begin{document}
 $$Q_n=\overbrace{K_1\ K_2\cdots\ K_2}^{\text{ بار}n}$$
 \end{document}