OK, if you are calling the automatic transmission the differental then you check the fluid level with the dip stick in park and the transmission warmed up. to drain the fluid out you have to remove the transaxle (transmission) drain pan located on the bottom of the transaxle. Do this by remove the bolts leaving two or three and crack the other end loose and let it slowly drain into a big pan. watch out for gushing cause there is alot of fluid in it (at least 10 or 12 quarts). You will most likely get a kit that contains a filter, a gasket and sometimes a plastic adapter. Once the pan is removed you pull the filter and replace it with the new one. Most of the time the old gasket is resusable )providing you clean it, the pan and transmission mating surfaces)if you didn't damage it or the seals are not cracked, flat or hard? It is very important to torque the transaxle (transmission) pan bolts in sequence (all bolts installed and hand tight starting from the middle and working out to each end gradually increasing the torque till you get to the limit (example hand tight, then 30 in pound, 80 in pounds, then 125 inpound or the torque limit)(foot pounds would be less cause each pound is 12 times higher than in pounds. so you would do hand tight, then 5 foot pounds then 12 or torque limit) and to the proper torque or the pan may develop a leak. I used an Inch pound torque wrench IMO they are 12 time more acurate than foot pound torque wrench. usually the torque is around 10 to 12 foot pounds (120 to 144 in pounds). I measure the amount of fluid I remove so I got a good idea how much to put in minus a quart (usually take at least 8 to 10 quarts. But don't over fill it or you will be remove the pan to drain some out or use a suction pump to get it down beteen the XXXXXXs on the dip stick.