Where is the plug and pan to change the transmission fluid on a 1990 Oldsmobile Cutlass?

On the bottom of the transmission. May or may not have a drain plug. Draining and replacing this fluid will only get about 25% of the fluid in the system removed. The fact that you ask this question, leads me to one conclusion. Take this to the dealer and let them change it. Almost all automatic transmissons on American made cars do not have a drain plug (like the engine oil drain plug). These systems are sealed and not really made to change the oil since few problems are associated because of the oil. Some auto part shops offer suction type devices to suck the transmission oil out thru the fill tube of the transmission. The most comman way is to jack up the vehicle from the front end, and then remove the lover pan underneath the transmission. Be careful since the pan will contain the oil used in the transmission, There is no other way to drain the oil before removing the pan. Since you will have access to the filter, replace the auto transmission filter that can be obtained at your local auto parts store. Once the oil is removed, replaced the seal (gasket) that also can be obtained from the local auto parts dealer. Refill with suggested required amount in your auto manual or ask your auto parts dealer the amount required for refilled.