To edit a page, open the page using the Asset Tree and click the "Edit" tab. If there isn't one, you don't have permission to edit that page.

The edit tab while viewing a page is highlighted

The page editor is very similar to Word. Hovering over a button will show a tooltip that describes each button.

News 6.8 Feature: Drop down Toolbar menu - Use this feature to easely edit pages.

Dropdown tool bar