The most common way to edit a page, assuming you have logged into the site already, is to navigate to the page using the public site menu and click the edit button in the lower right corner:
You can also use the content view to find then edit a page.
Once you have entered edit mode by clicking the edit button, you edit the page content within the Body section of the page. The Body section uses a simple editor and allows for:
Whenever you edit the content you should make notes about what you changed. This will help everyone who has access to the site easily and quickly determine what changes were made. Use the Publishing Options section on the left side of the editing screen, and provide a brief description of the main changes made to the page.
This will also make it much easier to figure out what has changed if you ever need to revert to a previous edition of the page.