How to rename theorem (or lemma) in Beamer?

You can redefine the theorem begin, and theorem end templates. In the following example, I suppressed the numbering and the parentheses, and added "of" between the theorem name and the theorem additional note:

\documentclass{beamer}

\makeatletter
\setbeamertemplate{theorem begin}
{%
\begin{\inserttheoremblockenv}
{%
\inserttheoremheadfont
\inserttheoremname
\ifx\inserttheoremaddition\@empty\else\ of\ \inserttheoremaddition\fi%
\inserttheorempunctuation
}%
}
\setbeamertemplate{theorem end}{\end{\inserttheoremblockenv}}
\makeatother

\begin{document}

\begin{frame}
\begin{theorem}[Fermat]
Test theorem.
\end{theorem}
\end{frame}

\end{document}

You can use the block env.

\begin{frame}[fragile]\frametitle{}
\begin{block}{Any Title}
  Simmons Hall is composed of metal and concrete.
\end{block}
\end{frame}