How to adjust shutter speed for Nikon D90 while shooting in P mode?

I've borrowed a Nikon D90 and am having a little trouble figuring out how to change the exposure and shutter speed while shooting in P mode. I don't want to shoot in Aperture Priority or Shutter Priority.

Spin the rear dial. This will provide an override to the program settings. To get out of the flexible program mode, you need to turn your camera off or switch to another mode.
In your manual, see page 80, Flexible Program.

The better way to do this is set the camera to shutter priority or manual mode.

P is programmed Auto mode.

IF you want to alter the settings then you DO have to shoot in Shutter priority or Manual.

Edit: turning the rear dial will alter shutter speed AND aperture

If you just want to control shutter speed, then shutter priority will automatically calculate the aperture for you for a 'perfect' exposure. Simples. I don't know what your reasons are for not wanting to use this mode, as it's very easy.

If you are using program mode, then you set the over or under-exposure of the photograph, moving the control dial changes how many stops you are over or under exposing, and automatically decides what aperture and shutter speed to use.