A green edit pencil can be found to the right of each section of a page (and some pages have a main "edit" button in the brown section at the top of the page). To edit pages you need to first login or create your free account via the green and blue buttons at the top right.
Clicking on an 'edit pencil' will bring you to edit mode with a window displaying the text or wiki text/code in that section (or the whole page if you clicked the brown main "edit" button). You can add, delete, or change text within this window and then save.
If it is a wiki page or section that you're editing you will have the following options: 'Show preview' to preview changes, 'Show changes' to show the "before and after" versions, and 'Save page'. (See image at below left.)