Changing the fluid is messy because most pans don't have a drain plug, so once the seal is broken, the fluid spills out. The pan is held to the trans by bolts around its outer perimeter. The trick to lessoning the mess is leaving one or two bolts in one side of the pan, and then tilting it towards a drain pan after the seal is broken. It's important to use the fluid specified by the manufacturer. This information can be found in the vehicle's manual. The transmission filter is bolted to the bottom of the trans.