To the right of a page's title, there's a bright green 'Edit' button you can press to update a page. Click it now. Welcome to the editor screen! There are two sides to it: Visual and Source. They are reflections of each other, as if you're staring into a distorted mirror, yes. 'Visual' is the dressed-up face and 'Source' is the frank face of a page. You can switch between them by clicking the upper brown 'Source' and 'Visual' tabs.