Navigate to the article by following a link or searching for it. Then click the “Edit” button, either at the top of the page to edit the whole article, or further down the page to edit just one section. The graphic below shows a hypothetical article with the "Edit" links circled:
The text editor will appear. Then just type your update and/or make any other changes that are necessary. When you are finished editing, click the “Publish” button at the bottom of the screen.