According to the memoir
user manual, a chapter title is set via \printchaptertitle
. An example of its usage and redefinition for the bringhurst
chapter style (section 20.4.1 The chapter style, p 354) is:
%% Bringhurst chapter style
\makechapterstyle{bringhurst}{%
\renewcommand{\chapterheadstart}{}
\renewcommand{\printchaptername}{}
\renewcommand{\chapternamenum}{}
\renewcommand{\printchapternum}{}
\renewcommand{\afterchapternum}{}
\renewcommand{\printchaptertitle}[1]{%
\raggedright\Large\scshape\MakeLowercase{##1}}
\renewcommand{\afterchaptertitle}{%
\vskip\onelineskip \hrule\vskip\onelineskip}
}
It's obvious how \printchapterstyle
has been redefined to set the contents in lowercase. In a similar vein, you're most like after
\renewcommand{\printchaptertitle}[1]{\chaptitlefont\MakeUppercase{#1}}
Here is a complete MWE:
\documentclass{memoir}% http://ctan.org/pkg/memoir
\renewcommand{\printchaptertitle}[1]{\chaptitlefont\MakeUppercase{#1}}
\begin{document}
\chapter{Sample Title}
\end{document}