To upgrade to a MkIV you only need to purchase a MkIV nav drive. It's not a software update, it is a replacement of the drive unit that is located in the rear electronics bay (the unit you put the the nav disk into).

In order to do the upgrade you must remove the MkII and the GPS receiver (the GPS receiver is built into the MkIV) and connect the cables to the MkIV instead.

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