Have not done it in about 3 years already, so do I need to have it flushed or just replace drain and refill? If so, how many gallons of coolant do I need to flush it down?

I also could not find the drain plug in the engine by looking at the top of the engine. Has anyone found it or can I just drain the lower radiator hose after removing the skid plate underneath the radiator.

Thanks for all the reply.