^Don't worry, it's simple to update once you know how (There's probably a tutorial being made by staff that's on the way)
Assuming you mean 'add a chapter' when you say update, here's what you do:
In the
story overview page, find the story you wanna update and click the 'Chapters' button
here. Then you get taken to the chapter overview, where you'll see all your current chapters. Then you click the 'New Chapter' button to the right,
here. Then there'll be a page very much like the old mibba where you can put in a title, your chapter, the author notes, and hit submit!