Choose Paint Edit > Modify Layer > Horizontal Flip   or Paint Edit > Modify Layer > Vertical Flip
  or Paint Edit > Modify Layer > Vertical Flip   .
 .
         
To flip an image layer horizontally or vertically
 to open the Canvas
                     Layer Editor.
  to open the Canvas
                     Layer Editor.
                from the Palette, or choose Edit
                           > Horizontal flip in the editor’s menus.
  from the Palette, or choose Edit
                           > Horizontal flip in the editor’s menus.
                      from the Palette, or choose Edit
                           > Vertical flip in the editor’s menus.
  from the Palette, or choose Edit
                           > Vertical flip in the editor’s menus.