After you click on the crop tool look at the bar at the top that has your crop ratios. There will be three boxes.
One that has the overall ratio, say 2:3 (4:6).
Then a second that has the vertical ratio, 2 in this example.
And a third that has the horizontal ratio, 3 in this example.
Clicking the arrows inbetween the second and third boxes will invert ratio and so make your crop vertical. Or alternatively you can just put your own values in the boxes. ie switch the 2 and the 3 around.