Any user can edit any page on the Codex, consistent with the functioning of a normal wiki. You can edit any page you come across here if it has an Edit button at the bottom of the page content area. You can also mouse over the dropdown icon in the top-right of the page area to access an edit link there as well.
Section Edit Icons
In the dropdown at the top of the page, there is a toggleable menu option for Edit Icons, which turns on puzzle piece shaped icons next to each section header (like the one you see above). This allows you to focus your page edits on a single section of a page.
Editing Procedures