How do you advance the timing on a 93 jeep cherokee 4.0. I can't get the distributor cap to turn

If you look at the distributor, you will see that the hold down is a bolt through an "ear which is part of the casting of the distributor housing.
By design, it is not-adjustable. Actual timing changes are made by the engine control module.
I had a problem with my 95, that seemed like symptoms of just about everything else except timing. After checking everything else twice, I removed the distributor, ground off part of the "ear" and used a smallblock chevy dist holddown, making timing possible. It worked!
However this solution indicates that my timing chain is worn beyond the parameters of the computer. Therefore, as soon as Spring arrives, the chain & gear will be changed!
If you want to make timing adjustment, don't. But if you must, it is possible!