96-97 v6 overheating. check engine light says cooling.

the car has antifreeze in it but it is pretty brown and nasty. it has 114,000 miles. it over heated on the way to work today. its at my work's parking lot. should i try to change the fluid? or the thermostat. if its the thermostat i would appriciate anyones help in finding it. i see the main coolant hose and poped the cap off but the thermostat was not visible so i think it is probably closer to the block. do i have to take the plenum off? or just take the plastic and shit off. any help would be great. im going to try to do it in the morning.

also when i got it scanned the guy said check the thermostat or the sensor, if the coolant wasnt low and there was some in the tank.