How to chage content in HTML pages?

Hi I am creating a web app for my project for that I have downloaded the Template and its fine.

But how can I make it so that change in one page can affect all of the pages?

Ex. In menu I havn't added "about us" and then after developing all the static pages if I want to add "about us" then how can I do using single change without going into all the pages and manually add it?