When you have a KeepKey attached, from the Tools screen select "Change PIN”.
The KeepKey device will show a PIN matrix on its screen. To enter a new PIN you must look at where the digits of your PIN are on the KeepKey screen and then click the corresponding button on the MultiBit HD screen. This "digit position" technique prevents electronic eavesdroppers from discovering your PIN.
You can repeat digits in your PIN and you can make it up to 10 digits long.