How do you replace oil pump on 1994 Toyota pickup 22r?

You'll need to remove the oil pan. That usually requires that you disconnect the engine mounts and lift the engine a few inches. Once the engine is up, pull the pan bolts and the oil pump is connected to the screen that's hanging down. Unfortunately, you probably won't be able to improve the oil pressure by changing the oil pump. If the engine is running but has low oil pressure it's probably caused by worn main and/or rod bearings. Rather than changing the oil pump, try a slightly heavier grade oil. If you've been using 5W15, change to 10W30 or 15W50. Whatever you have been using, just go up a number. If that improves your oil pressure, it's definitely not a problem with the oil pump.