The Toyota Previa has a wonderful but strange flat-four engine that is located underneath the vehicle. If you open the hood, you see a bit of the cooling system, but really only the fluid addition reservoirs.
To change the oil, you need to have access to the underside of the car and to pivot the driver seat back. The seat release is somewhat hidden at the front center (usually, a yellow colored lever). Pivot the seat back, move the carpet out of the way toward the center, and undo the engine-cover knob. Pivot the cover up and off. You will see the oil addition cap toward the left and the dipstick toward the right.
To change the oil, remove the oil addition cap and the oil-pan bolt (bottom of the engine). Be sure to have a recepticle to catch about 6 liters of oil. Properly dispose of used oil. After the drip rate has gone down to about 1 drop/min (to hasten this, it's best to have the engine warm/hot), replace the oil-pan plug; make it tight but don't overtighten it. Also, remove and replace the oil filter (e.g. Champion OF-10); you'll find it facing forward, further toward the front-center of the Previa. Add oil to the oil addition port above (it will take 5-6 liters; I usually use 10-30). Replace the oil addition cap, put the engine cover back in place, tighten the engine cover, replace the carpet, and pivot the driver seat back (be sure this is properly seated!). You're done!