STEP 1
From the Library or Develop view, Press ‘R’ to activate crop mode
STEP 2
Press ‘X’ to change the crop aspect ration from portrait to landscape or landscape to portrait.
STEP 3
Press ‘Enter’ to commit the crop.
STEP 4
Export your file.
STEP 5 (optional)
You may want to reset it back to original after. Just press the ‘Reset’ button so the original aspect ratio shows in the library.