In paperspace go into the viewport you want to rotate.
Make sure your UCS icon is turned on, so you can see what's happening, just type UCSICON and select ON.
Type UCS and then Z and enter your amount of rotation, usually 90 or -90.
The 'Z' axis is perpendicular to the 'X' and 'Y' axes and orientated so that it's pointing out of the computer screen towards you, hence the rotation flat with your screen when you rotate about the 'Z' axis.
Now type PLAN and select CURRENT.
This should rotate your drawing in the viewport and give you the equivalent of rotating the viewport.
Dear ControlConcepts,
The above answers are both correct, also you can try the following method:
1- Set the WORLDVIEW system variable to 0.
2- Using the UCS command set the ucs in desired direction so that the X axis lies in horizontal direction for viewport to be rotated.
3- Enter the VPOINT command and set the view direction to 0,0,1 .
Farzad