The only way you can change the analogue display(speedo) is by overlaying a kmh sticker on the dial or changing the whole unit.
However, you can change all the digital readouts eg mpg, average speed, average distance etc using an obd11 reader and an app called Motoscan. You need a connector cable from the diagnostic plug under the seat, the OBD11 reader and app all at a cost of about £75 unless you know someone with aGS911 tool.
I think the dealers will do it for about£120 but not the speedo.
Just done mine the other way round - km to Miles