How do you replace wiper arm on 1999 dodge caravan?

If you mean the blade it is a small tab at the end of the wiper arm. When you buy the new blade the rep behind the counter will glady show you how it is done....Takes about a minute.

If you actually mean the wiper arm it is also fairly easy. Where the arm attaches to the vehicle (pivot point) there is a small cover on the wiper arm over the top of the hold down nut. Using a scribe or small flat blade screwdriver you can pop the cover off to expose a 13mm nut (maybe 15mm) that you turn counter clockwise to loosen. Once the nut is off you can move the arm back and forth with slight upward pressure until it pops off. Some of them will have corroded a bit to form sort of a weld. If yours will not come off as I described put the nut back on just far enough to protect the threads on the pivot stud. Take a hammer and gently (not too gently but carefully) hit the top of the nut and try removing it again. Continue until it comes off but, be careful with that hammer.