I had to replace the instrument cluster with a used one, obtained from a used parts dealer. The replacement unit has a large difference between the actual mileage of the vehicle vs the one shon on the dash. Can anyone advise me on how to change the odometer reading to reflect the actual mileage? What procedures are needed and can I do this myself

Can you fix the discrepancy yourself, no.

Newer cars store the odometer value in at least 2 modules, cluster and ECM, and in some cases even the BCM. When replacing one of them, a diagnostics routine for handshake is required ("seed and key") to have the replaced module learn data about the car, being the odometer value one of them if applicable. I'd advise taking it to the dealership to see what assistance they offer.