You can edit a page by clicking on the third tab from the left, at the top of the page. It's titled "edit", sensibly enough. If there is no "edit" button, that means the page is locked against changes - it's either protected like the Main Page, in which case the tab is replaced by one that says "view source". It's like the edit button, but you cannot save any changes you make to the page source - it's read-only. Or it's a Special page, in which case there are no tabs at all.
Once you have brought up the editing form, you can find the place you want to change and make your modification. In the Summary field below, explain in 2-3 words what you did ("Categorized", "Typo", "Broken link", etc) and click save. Ideally, you should preview first - if you make a typo, it requires a further edit, and that wastes space in the database.
For information on Wiki markup, you can read the Editing help here.