Radiator Flush & 180 Thermostat

[megax29]

My car has been heating up lately and I have never changed the coolant on my car. It has 140k miles and had a radiator replacement when I hit someone at 50k, so I know it must have been changed then. I don't know why it's heating up but I notice my AC sometimes doesn't get cold either...not sure if they're related. I've decided to flush my radiator and change my thermostat to 180. I read on the capacity of my radiator and it says it's over 12quarts or 3 gallons and I can't seem to get all of it out. I unscrewed the plastic cap on the passenger side of the radiator and opened the radiator cap. When I pour water in the cap it does come right out but I suspect there's some in to motor and I see some in the resovoir between the motor and the fan. I've read about a valve I should open to get all of it out, where is that located?

[Kiljosh]

There are two drain plgs on either side of the engine block. Once you take them out all the coolant will empty out. They're white, maybe 1/2" diameter