I can access to the back-end cms area of a website from a web browser. I want to make changes to pages from back-end. Before doing so, I'd like to backup the site by copying all the folders. How can I do it? Thanks.

You can simply use this plugin:

http://octobercms.com/plugin/renatio-backupmanager

or you have to manully save your web root folder for full backup. If you only making chances in at cms files or static pages its enough to save your active theme folder. If change other datas mostly db changes occur. So you also have to make a mysql backup. For example via Phpmyadmin.

Thanks for the reply. I cannot access to the root folder. I'm accessing backend CMS from a web browser.