It's extremely easy to change the coolant. Once you've drained the coolant, it's a good idea to change anything else that's old since it's super easy to do at that point. I recently changed all my hoses (they were all original) as well as the thermostat, cleaned the heat exchanger, and replaced several gaskets and o-rings. Working on these engines is a breeze! The hardest part was figuring out the correct part numbers and waiting on parts. I spent about $300 in parts when all was said and done to do a decent overhaul of the cooling system.
On the 2GM20F there are two small hoses on the side of the engine that drain the coolant. I drain those into empty water bottles, then dump them into an empty 1 gallon milk jug for disposal. Next, I filled up with distilled water and ran the engine for 15 minutes, then flushed it out. I repeated this process of flushing out several times. If you're needing it, you could plug the outlets for the water heater hoses, and take the time to replace those two hoses. I didn't do this, and I ended up needing to drain the coolant again not long after I did the flushing.
Then fill up the top with coolant. When you start the engine, you'll want to loosen each hose clamp and squeeze the hose a bit until coolant runs out to confirm that you've worked out all the air trapped in the system. It's a similar process to bleeding the fuel, you'll definitely want to bleed the coolant at each hose. My engine started to overheat when I first changed coolant because I had trapped air in the hoses. For the first couple minutes after starting the engine, keep topping off the system, then replace the radiator cap, and that's when you start to crack open hose clamps burb the air out of the system.
The biggest problem seems to be trapped air in the system. Another tool you'll want is an IR thermometer. As the engine is running, you aim the IR gun at various parts of the coolant system and engine, and note the temperatures. I run around 165-180 for different components of the coolant system and engine.