Not positive the answer is the same, but here is the answer for the Rev; it might be the same. The same thing happened to me after a buddy replaced my top end and put stuff back together -- there is a single black wire that goes into a single connector up near your gauges. Unplug it, and your speedo should go back to mph.
Hope that works for you...
EDIT: re-read your post; I assume you mean you want to change it back to mph.