Piece of cake, but you'll need a timing light to adjust spark timing.

Unbolt the distributor, probably 3 bolts. Notice the blade on the end of the distributor shaft is offset a little bit, so it's only supposed to fit one way.

Put a little oil on the new distributor's O-ring so it doesn't tear when you put it in. Spin the new distributor's shaft so the offset blade fits. If you get it backwards, it IS possible to force it in. It should slide in nicely.

Adjust your spark timing. I think the timing marks are on the flywheel, so you remove a rubber plug in the bellhousing. Hope someone chimes in, because I'm not real sure about the timing on a '90.