When you edit your submission after uploading it, there will be three tabs at the top called "Map", "Media Gallery" and "Updates". Go to the Updates tab, name and write your update log, save it, go to the Map tab and save your submission as a whole.
But maybe make sure that you don't create an update log when you didn't actually change your submission cause that would be like "Hey, I've got something new" when there isn't anything new in reality.