As already said you have to use \makeatletter and \makeatother because in Tex @ ist used as special character for internal commands. If you really want to change the format this way you have to change the defaults. Here is a minimal working example:
\documentclass{article}
\makeatletter
\renewcommand\section{\@startsection {section}{1}{\z@}%
{-3.5ex \@plus -1ex \@minus -.2ex}%
{2.3ex \@plus.2ex}%
{\normalfont\Huge\bfseries}}
\makeatother
\begin{document}
\section{test}
\end{document}
It might be easier to use a special package like titlesec
\documentclass{article}
\usepackage{titlesec}
\titleformat{\section}{\Huge\bfseries}{\thesection}{1em}{}
\begin{document}
\section{test}
\end{document}
or sectsty
\documentclass{article}
\usepackage{sectsty}
\sectionfont{\Huge\bfseries}
\begin{document}
\section{test}
\end{document}
Or if you are using Koma Script you can eaily do:
\documentclass{scrartcl}
\setkomafont{section}{\Huge\bfseries}
\begin{document}
\section{test}
\end{document}
I wouldn't recommand to change the section format directly. I would use Koma-Script or titlesec
\makeatletter..\makeatother. – Martin Scharrer Jan 04 '12 at 10:07\makeatletter...\makeatother? See http://tex.stackexchange.com/questions/8351/what-do-makeatletter-and-makeatother-do. – lockstep Jan 04 '12 at 10:07\sections are\Largeby default in the standard document classes. – lockstep Jan 04 '12 at 10:45