\AtBeginDocument{\renewcommand{\chaptername}{Section}}
\usepackage{fancyhdr}% fancyhdr related command; details in its documentation
\pagestyle{fancy}
\fancyhead{}
\fancyhead[LE,RO]{\thepage}
\fancyhead[RE]{\slshape\nouppercase{\leftmark}}
\fancyhead[LO]{\slshape\nouppercase{\rightmark}}