Change newtheorem numbering order in Arabic : from Right to Left

You don't provide a MWE or say what packages you are using. In case it's babel, you may need the option layout=counters:

documentclass{article}

usepackage[bidi=basic, layout=counters]{babel}
babelprovide[import, main]{arabic}
babelfont{rm}{FreeSerif}

newtheorem{thm}{مبرهنة}[subsection]

begin{document}

setcounter{section}{3}
setcounter{subsection}{2}    

begin{thm}
   ABC.
end{thm}

begin{thm}
   DEF.
end{thm}

end{document}