Add Toyota coolant designed to best protect your aluminum radiator and head and / or block, mixed with 50 - 60% distilled water. If the mixture in the overflow tank is old and dark take the opportunity to drain the radiator and re fill. You need to buy the gallon of coolant anyway, and when you add distilled water, you'll have two gallons or more. If you have the 4 cylinder, you should be able to reach the plastic drain plug at the bottom of the radiator from above. It only needs to be hand tight. Remove the radiator cap and then open the drain at the bottom. Let it drain into a pan (I use a plastic concrete mixing tub), then dispose properly and slowly re fill using a small plastic funnel you can find at a grocery store. If you want to go one step further, as preventative maintenance, buy a new Toyota radiator cap when you get the coolant, and just replace it when you change the coolant. The simple method is just to fill to the full line on the reservoir, but if the coolant is old, you'll be doing the engine a favor to drain the radiator, too. If the engine protecting additives in the coolant wear out from being left in too long, the internal engine passageways in the water jacket can begin to be coated with corrosion or contaminants, making the engine run hotter, even though you may not see it on the temperature gauge. Localized hot spots put more strain on the oil, especially conventional oil, will thin and oxidize more rapidly at higher temperatures, which can leave deposits that don't drain out with an oil change. This can also create localized hot spots within the engine. Just generally a good idea to keep the inside of the engine as clean as possible if you plan on keeping the car a long time. Toyota recommends also removing the coolant drain plug(s) on the engine block, but when I did this on the 4 cyl. only a few drops came out, so I'm not about to go through that again. Properly maintained Toyota coolant also helps the water pump last longer. If you're really concerned about spending the few dollars to buy coolant, distilled water and a radiator cap, most Auto supply stores sell an inexpensive gauge (around $3) with a rubber bulb to suck up a small amount of coolant to test the remaining antifreeze protection.