I can't find a drain plug or petcock on the radiator of my 2002 century

Some radiators don't have a drain. You don't want to use it even in you have one. They always break when you try to close them. Remove the lower radiator hose instead

https://ricksfreeautorepairadvice.com/radiator-drain-valve-location/

It might not look exactly the same, but it will be similar to this one (which is a direct replacement)
https://www.carparts.com/details/Buick/Century/Dorman/Radiator_Drain_Plug/2002/Limited/6_Cyl_3-dot-1L/RB61136.html

on the bottom off rad shound be a drain plug dr side

Hi, if there is not a petcock or plug at the bottom of the radiator, you can just remove the lower radiator hose to drain the coolant. Many cars do not have drain plugs. Please let me know if you have more questions and thanks for using FixYa.

Made a video showing how I replaced the radiator on my 2005 Hyundai Santa Fe and in the process I show where the drain plug is.

take the bottom hose off from the radiator

Should be a drain plug or petcock at the bottom or rear of the radiator or remove the lower radiator hose. If there is a petcock just/drain plug. Generally on this year you would rotate it 1/4 turn and it will start draining. If it doesnt drain there is something inside the radiator. Then you will have to resort to removing the lower radiator hose. If you need further assistance please let me know. Thanks, Midwest-tek

Generally, when one wants to evacuate the system of the radiator fluid, one can remove the bottom hose in order to quickly drain the system, much faster and more minerals comeout that way than if you use the drain plug which is very small.

There should be a drain plug at the bottom of the driver side tank of the radiator. Unscrew that and remove the radiator cap.

Drain about 1/4 of your radiator fluid into pan with petcock under radiator. Then follow upper radiator hose and it will lead directly to the thermostat housing. It should have about 3 bolts on it. Just pull off hose, then housing. Thermostat will pull right out. You may or may not get a gasket with new one. If not, you can use RTV silicone for $3-4 and spread a thin layer of silicone around between housing and surface, then reconnect everything. Let dry for couple hours, refill radiator while veheicle is running and make sure thermostat opens. When open, coolant level will drop and you should add more til full. Then your done.