Just like the spark plug the distributor cap and rotor needs to be replaced on a regular basis.  You should replace it every other time you replace your spark plugs.  Since my 993 have accumulated 28,000 miles I feel now it is a good time to replace it.  Most newer cars now days have electronic ignition without distributors, the reason why Porsche still uses the mechanical distributor system for the 993 is unknown.  But Porsche changed to the newer distributor less system on the 996. 

If you have any doubts about replacing your distributor cap, take a look at this below picture of my distributor cap at 28,000 miles.  The picture clearly shows the oxidation and the arching damaged from the high voltage sparks.  Click on the picture to get a closer view.

First step in replacing the distributor cap is to remove all the ventilation ducts that is blocking your way.  First step is to remove the two 10 mm bolts that hold the upper duct in place.

Pull the the temperature sensor from the upper duct housing by squeezing the metal clip.  Remove the upper air ducts.

Loosen the 1st strap on the bottom duct

Loosen the 2nd strap screw

After removing the heater coil element and the white robber ventilation hose for the distributor belt the lower air duct can now be completely removed

The distributor is held on by two philips screw, the screw is on very tight so be careful not to stripped the screws.  I stripped the lower screw on the bottom distributor cap and had a hell of time removing it.  Actually I had to remove the muffler, the muffler mounting bracket, remove the engine shroud, remove the heat exchanger pipe mounting screw that goes into the ventilation pipes.  It was not fun.  Since I was planning to change out the spark plug next I had to take most of these parts off any ways. But you want to be careful on the lower screw on the bottom distributor, it is at a awkward angle to get to.

Update 8/15/03 
Bruce SEA 993 suggested that if you try to tighten the screw prior to unscrewing it will prevent any stripped screws.  By tightening it prior you break loose the torque that has been built up on the screw.

Here is a picture of the stripped screw

After loosen the distributor screw you first remove the cap with the spark plug wire still on it and replace the rotor that is underneath it.  There is no trick to that just pull off the old one and push in the new one.  There is a notch on the distributor shaft so there is no possibility of putting in the rotor incorrectly.

Place your new distributor cap next to the old one and replace the spark plug wires one by one, I know there is very little room to work with there, but I did it like that.  This way you eliminate any possibility of hooking up the spark plug wire to the wrong post.