How do I adjust a mixing valve?

My standalone shower has a Kohler mixing valve. Over time it has become harder and harder to set the shower on hot -- now if you turn the valve all the way to hot, the flow is lukewarm at best. The hot water supply elsewhere in the bathroom is fine, and I suspect the mixing valve is out of whack. Is there a way to adjust the valve without tearing up the wall?

Peter
March 2008