Changing antifreeze

If the system is not contaminated, and you just want a refresh, a simple drain and fill is sufficient. Your radiator does not have a drain, I guess because GM was cutting cost at the time. Here is the procedure. Typical concentration is 50/50 water/dexcool.

Draining Procedure

Caution: To avoid being burned, do not remove the radiator cap or surge tank cap while the engine is hot. The cooling system will release scalding fluid and steam under pressure if radiator cap or surge tank cap is removed while the engine and radiator are still hot.

Important: Do not use coolant system sealers in this cooling system.
†Park the vehicle on a level surface.
†Remove the surge tank cap.
†Remove the air box. Refer to Air Cleaner Assembly Replacement.
†Raise and support the vehicle. Refer to Lifting and Jacking the Vehicle.
†Remove the air deflector. Refer to Front Air Deflector Replacement.
†Lower the vehicle.
†Place a drain pan under the lower radiator hose connection.
†Using the J 38185 reposition the lower radiator hose clamp away from the radiator.
†Slowly remove the lower radiator hose. Drain the coolant into the drain pan.
†Inspect the engine coolant for the following conditions:

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Discolored appearance--Follow the flush procedure. Refer to Flushing.

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Normal in appearance--Follow the filling procedure.

Filling Procedure

Notice: The procedure below must be followed. Improper coolant level could result in a low or high coolant level condition, causing engine damage.
†Install the lower radiator hose.
†Using the J 38185 reposition the lower radiator hose clamp to the radiator.
†Lower the vehicle.
†Slowly add a mixture of 50/50 DEX-COOL antifreeze and deionized water to the cooling system through the top of the surge tank opening. Refer to Approximate Fluid Capacities.
†Fill the surge tank to the FULL COLD level.
†Install the surge tank cap.
†Idle the engine for 2 minutes and occasionally raise the throttle to 3,000-3,500 RPM.
†Turn off the engine and allow the engine to cool.
†Repeat the steps 3-7.
†Install the air deflector. Refer to Front Air Deflector Replacement.
†Install the air box. Refer to Air Cleaner Assembly Replacement.
†Allow the engine to cool.
†Remove the surge tank cap and fill to the FULL COLD level.
†Use the J 26568 to inspect the concentration of the engine coolant.
†Rinse away any excess coolant from the engine and the engine compartment.