How do I refill water into the radiator of a 1998 Pontiac Grand AM SE? I do not know where the refill cap is?

Two things to check:

1. Make sure you do not have an air bubble in your cooling system. Easiest way is to park facing uphill and let the car cool off. Remove the radiator cap, start the car and let run until the thermostat opens. If there are air bubbles in the cooling system the should be forced out. Top off fluid.
2. Are you sure that the cooling fan is running? When the car is getting hot, open the hood and make sure the radiator fan is actually turning.

Terrific question!
First, fill the radiator with coolant, do not fill to the top, just fill about halfway up in the tank.
Start the engine and turn on the heater full blast.
Probably would be good to give it some gas to about a medium idle and hold it there, might require 2 people.
Very carefully watch the level in the tank.
DO NOT put your face over the hole. When the thermostat opens , the coolant will drop quickly.
Try to keep adding coolant as that happens.
After about 5 minutes you should be ready to go.
The way you will know is the heater is blowing hot air.
Now fill it up and replace our cap.
Be Blessed.

Open radiator drain & coolant cap

Drain coolant----close rad drain & refill

50% Coolant 50% Distilled water

follow upper radiator hose to aluminum housing this is where the thermostat is. be sure engine is cool remove radiator cap or reservoir cap if no cap on radiator disconnect upper hose from thermostat housing. remove two bolts in housing and remove housing. replace thermostat and reassemble. when finished refill anti- freeze leave cap off and start vehicle with heat on high. run the vehicle like this until air stops bubbling out of coolant and heat is hot steadily. continue topping off coolant during this process and be sure cooling fans engage before completion. then youre all done

You do it thru the overflow-reserve tank. That is the pressure cap normally found on the radiator.

Remove the serpentine belt from the water pump pulley. Remove the lower radiators hose's upper clamp at the overflow tank. Drain as much of the coolant as you can. Saving the liquid in a drain pan can be a good idea. Use a ratchet with a 10mm socket to remove the heater line off the top of the water pump. Remove the bolts holding the water pump body. Remove the old pump and clean surfaces completely. Install new pump with new gasket and silicon sealant. Reverse procedure to complete, Refill radiator via fill tank, check for leaks. Continue checking overfill tank for a few days.

Fill the radiater completly then sart the car and let it warm up.After it starts putting heat out of the defroster the top up the rad and let it run a few more minutes then shut it off.Wait about a minute,top up ther rad and put the cap back on,then restart and top up the overfvlow bottle.

passenger side fender. it is a clear yellow jug with a black (radiator filler) cap

This is not an easy job for the do it yourselfer. You have to remove the timing chain and the exhaust manifold.

IT DOESNT RESET
smack the coolant jug a couple times with your hand the sensor is in there and will stick