Your current projects are automatically saved in the homepage of the app until you place the order.
Alternatively, while you are configuring the product on the "Personalise" screen, you can click the icon in the upper right corner: the project will be saved on the homepage as well.
After your order has been placed, you will no longer be able to view or edit your project. You can, however, review/reorder the project of an already placed order: you will find more details in the FAQ "How can I review and/or reorder a product that’s already been printed on the PhotoSì app?".