I wanted to put the title of the chapter on even header and the title of the section of the chapter to the odd pages.
My headers are now are using this code:
\begin{document}
\frontmatter % Use roman page numbering style (i, ii, iii, iv...) for the pre-content pages
\setstretch{1.3} % Line spacing of 1.3
% Define the page headers using the FancyHdr package and set up for one-sided printing
\fancyhead{} % Clears all page headers and footers
\rhead{\thepage} % Sets the right side header to show the page number
\lhead{} % Clears the left side page header
\pagestyle{fancy} % Finally, use the "fancy" page style to implement the FancyHdr headers
Moreover, chapters are in separate files and called in a main file.