change the itemize from bullet to square

Try this. It changes the font size just before typesetting the \blacksquare. Depending on what you need, other sizes, such as \tiny are also available.

\documentclass{article}
\usepackage{amssymb}
\begin{document}

\begin{itemize}
 \renewcommand{\labelitemi}{\scriptsize$\blacksquare$}
 \item  An extensively analysis
\end{itemize}

\end{document}

I use something like

\documentclass{article}

\usepackage{xcolor}
\begin{document}

\newcommand{\localtextbulletone}{\textcolor{gray}{\raisebox{.45ex}{\rule{.6ex}{.6ex}}}}
\renewcommand{\labelitemi}{\localtextbulletone}
\begin{itemize}
\item  An extensively analysis
\end{itemize}

\end{document}

so I'm able to change the size. The colour is, of course, optional.