How do you change an oil pump on a 98 town and country minivan?

First, it's almost always a waste of time. Low oil pressure is typically the result of worn main and/or rod bearings. But if you insist on replacing the oil pump, you'll need to drop the oil pan. That typically requires that you raise the engine enough to get it out of the way, meaning that you'll probably need to disconnect the motor mounts. Before you replace the oil pump, check the bearing surfaces by pulling a main bearing cap and check the surface of the bearing. If it's worn through the babbit surface, the problem is definitely not the oil pump. You'll have the pan off already, so checking the bearing surface shouldn't be much effort. If you're still in doubt, check a rod bearing as well. If the bearings are ok, make sure that you put the bearing caps back on using a torque wrench. If you just have low oil pressure, try using a heavier grade oil before you go to the expense and trouble of changing the engine. If you've been using 5W15, change to a 10W30 or even 15W50. If your oil pressure is very low. Keep going up in number until you've tried the highest. Good luck.