theorem numeration in appendix

Assuming you are using the standard book class, without further packages, one solution is to redefine @thmcounter at the time appendix is ​​declared:

usepackage{etoolbox}
  makeatletter
    pretocmd{appendix}{def@thmcounter#1{noexpandAlph{#1}}}{}{}
  makeatother

Here is a minimal working example:

documentclass{book}
  newtheorem{theorem}{Theorem}[chapter]
usepackage{etoolbox}
  makeatletter
    pretocmd{appendix}{def@thmcounter#1{noexpandAlph{#1}}}{}{}
  makeatother

usepackage{hyperref}

begin{document}
 
chapter{Lorem}
  See autoref{thm:2}

  begin{theorem}label{thm:1} 
    Hello world!
  end{theorem}

appendix

chapter{Ipsum}
See autoref{thm:1}  
  begin{theorem}label{thm:2}
    Hello world!
  end{theorem}
 
end{document}

which gives: