Editing is easy! Just click the "Edit" button at the top of the page. This will take you to a text area where you can edit the contents of the page. Once you're done, click the "Save page" button and you're done! Your edit is saved and live on the site. Be sure to preview your edits before saving, and practice on the sandbox first.
Sections of the page can be edited individually by scrolling down to the relevant header and clicking edit. The process is exactly the same, just you only see and affect one section instead of the whole page.