The \subsection
command for standard classes could be defined as
\titleformat{\subsection}
{\normalfont\large\bfseries}{\thesubsection}{1em}{}
You can just remove \bfseries
like
\titleformat{\subsection}
{\normalfont\large}{\thesubsection}{1em}{}
To make them un bold.
Another way is to use the short form:
\documentclass{article}
\usepackage{titlesec}
%\titleformat{\subsection}
%{\normalfont\large}{\thesubsection}{1em}{}
\titleformat*{\subsection}{\normalfont\large}
\begin{document}
\section{Some section}
This is some filler text.
\subsection{Some subsection}
This is some filler text.
\end{document}
Or you can also use sectsty
\documentclass{article}
\usepackage{sectsty}
\subsectionfont{\normalfont\large}
\begin{document}
\section{Some section}
This is some filler text.
\subsection{Some subsection}
This is some filler text.
\end{document}
If you want to fire an ICBM on a squirrel use the following:
\documentclass{article}
\makeatletter
\renewcommand\subsection{\@startsection{subsection}{2}{\z@}%
{-3.25ex\@plus -1ex \@minus -.2ex}%
{1.5ex \@plus .2ex}%
{\normalfont\large}}
\makeatother
\begin{document}
\section{Some section}
This is some filler text.
\subsection{Some subsection}
This is some filler text.
\end{document}
Here the original definition was \normalfont\large\bfseries
titlesec
package can easily do that.