When a selected area is created, select the transformation tool to transform the selected area. When there is no selected area, select the transform tool to transform the current layer.
In standard mode, drag fixed points to scale the selected area without change its aspect ratio. If you only drag one side edge, the selected area will only adjust that edge.
Stretch an item in all directions.
Flip the selected area horizontally.
Flip the selected area vertically.
Reset all transformations you've previously applied.