How do you crop a landscape photo to 4x6 portrait?

I am cropping photos to 4x6. If the original photo was taken "landscape", but I when I crop to 4x6 I want to make the final photo "portrait". How do you do this? When I crop to 4x6 with constrain, it stays landscape. There has to be a way to flip the cropped area to go up and down. I need it to do this to best crop the photo.

Someone please let me know how to flip this.

Thanks!