The crop tool has constrained and unconstrained modes to ensure that the final image is at the required ratio.
However I (and I realise this maybe me) cannot see a way to flip the ratio in the constrained mode to create a vertical/portrait crop in a Landscape image.
I can see that unconstrained allows a portrait crop but then there is no way to ensure the final image is 6:4 etc…
When you drag the corner of the frame, there are two positions to consider:
The corner of the frame.
The mouse pointer position.
Regardless of where the corner of the frame ends up, you need to get the mouse position sufficiently far to ‘flip’ the orientation.
If you have a 3:2 (wide:high) ratio (aka 1.5:1), you need to drag the mouse to a position such that it is more than 1.5 times as far away vertically from the opposite corner as it is horizontally.
Put way more simply. If you have a wide ratio, drag it very tall, and vice versa.