err, working strictly on memory of a 997 (might not be right so my apologies in advance) theres an option on the PCM settings, i think under settings under 'trip'. I could be wrong mind you.
From memory, using the bottom stalk on the LHS of wheel, push forward, go down to 'Set', push forward, you should see the kph and mph option, push stalk forward to set desired option, pull back stalk a few times to return to normal display.