No easy way I know of to adjust the mileage reading. (to make it harder to fraudulently alter)

If your car is a 90, the replacement cluster must be from an 89 or 90, or the speedometer won't work correctly (voice of experience speaking)

To remove and install the two knobs you have to remove the face from the cluster. There are several screws on the back that hold it in place. Remove these and you can swap them over.

Take the old one apart first to avoid making a mistake on the good one.

See the FAQ for further information on disassembling the cluster (which screws to remove etc.)