MWE:
\documentclass[lang=en,11pt]{elegantbook}
\usepackage{lipsum}
\title{Book Title}
\subtitle{Chapter and section at header alternatively}
\author{No author name given}
\institute{ABC}
\date{February 10, 2020}
\begin{document}
\maketitle
\frontmatter
\tableofcontents
\mainmatter
\chapter{First Chapter}
\lipsum[1]
\section{First Ection}
\lipsum[1-20]
\subsection{First Ection}
\lipsum[1]
\lipsum[1]
\end{document}
How can I get chapter name and section alternatively in elegantbook?