How does one change the transmission fluid in a 1989 Nissan Sentra standard 4 speed transmission front wheel drive?

Ok see if I can explain this to where you understand. It is very simple to do. You have to look at the transmission. So you probably need to jack up the vehicle to get underneath. Ok look for a bolt that looks like a square. it will not look like it is raised , but it will look flat like where you put a ratchet wrench into it. Some have a upper and lower one. you want the one closest to the ground. All you do it. take a ratchet and unscrew the screw and the fluid will drain out. There should be a filler plug up top somewhere. Best bet is go to Auto zone and buy a Haynes book for your Vehicle. Costs around 12-15 bucks. Not a great book , but has pictures and explains alot on maintenence of your Vehicle.