From the page you wish to edit, click on

Open

In the editor, use the links above the editing pane to switch between 'Rich Text' editing (WYSIWYG editor, similar to Microsoft Word) and 'Wiki Markup'.
NOTE: Switching between 'Rich Text' and 'Wiki Markup' editing may cause problems with page formatting. If you regularly use macros or extensive formatting, you may want to set your default editor to 'Wiki Markup'.

To see what your page looks like before saving it, use the 'Preview' link.

Once you are finished editing the page, click either of the 'Save' buttons.