Why would you be trying to change the mileage? Might want to clarify your intention first. Changing mileage on a vehicle is a serious offense and most guys would probably not be real willing to help unless you have a reasonable excuse.
Not the mileage on the truck. I mean when you get a new cluster u have to set it to how many miles are on the truck (i believe). Trust me I don't want to make it read wrong but read right
FYI...you can drive the truck with out the cluster...you just need to find a way (GPS) to tell how fast you are going. I drove mine around for a week without the cluster while I had it rebuilt.
I'm sure you can find a write up just do a search. The whole process is just soldering a new motor in...super easy. Taking apart the cluster and putting it back together as well as setting the needles correctly is the hardest part.