There can be several things that may be wrong.
The dash cluster itself may have gone bad. Try tapping on the top of the dash with the car running to see if you can get the problem to go away, change, etc. this would indicate a bad cluster. Not to say it can be bad and this test not do anything.
Another possibility is a bad Body Control Body Module (BCM). The BCM controls many dash gauge and indicator function, but not english to metric.
Really no way you can test it yourself without a scanner that is capable of controlling dash functions and looking at input data.