If you have a flat image without multiple layers, press "Ctrl-A" to select all, then "Ctrl-C" to copy the document to the clipboard. Select "Image Rotation" from the File menu and select a 90-degree rotation, either clockwise or counterclockwise, to rotate the image to landscape mode.
To change the orientation of the whole document, select Layout > Orientation. Choose Portrait or Landscape. Change part of a document to landscape Select the content that you want on a landscape page.