Edit the page by working on the text in the box directly, with basic text-editor insert, copy, cut, and paste functions. You could also copy text from other applications (I write many Wiki pages in sequence in a large TextPad file, then assemble the pages in the Wiki by copy-and-paste) into the editing box.
Under the editing box is a Summary box where you can (if you wish) provide a helpful description of the changes you made. This Summary appears in the Recent Changes displays. You could, instead, mark the This is a minor edit checkbox, to tell the system not to display the change for most searches, for instance if you simply corrected a typo or two. To tell folks on the "email when the Wiki changes" list that the page has been changed, click on the checkbox for Send email notification that ... has been changed.
Click on Save to save the changes and rebuild the page into format. If you are a uncertain of the effect of your changes, click on Preview. This action rewrites the page to add the text in full format to the editing page, below the editing box, so you can see what will happen when you save the page. When you are satisfied with your work, click on Save.
That's all there is to editing text in this Wiki. Of course, more practice with exact coding syntax (far simpler than full-blown HTML coding, if you wish) and writing-for-layout methods will improve the product you make.