You can update your diary every week by adding a week. But you did that already so I think that you want to know how to edit a week and add new pictures and text to this week when you got a new update after a shorter period of time then a full week.
You need to go to the page from the week that you want to edit and click on the 3 dots. When you click on the 3 dots a small menu will give you a few options for this week and 1 of them is edit this week. Here you can continue adding pictures and text to that week.