By default, a canvas or poster collage is displayed in landscape mode. If you want to show and edit it in portrait, rotate the work area in Editor (icon at the bottom in the view options) and then rotate the photo or the photos in the relevant photo placeholder in the opposite direction (icon at the top in the "Image" toolbar). There are new layouts that have been created already in portrait mode. If you choose one of these layouts, the view of the poster automatically rotates on the desktop.