documentclass[envcountsect]{beamer}
\usepackage{amsthm}
\usetheme{Warsaw}
\setbeamertemplate{theorems}[numbered]
%\setbeamertemplate{theorems}[ams style]
\begin{document}
\begin{frame}
\frametitle{title}
\begin{theorem}
This is a theorem.
\end{theorem}
\end{frame}
\end{document}
