Usually there is a petcock on the radiator. You can drain (remove cap to let air in) and refill.
Supposedly it lasts 10 years now, so your due.
Be careful draining it if there are any pets around, it is poisonous and it smells sweet to animals.
You can buy the premix or the full strength and mix 1:1 with distilled water. Usually need 2 gallons.--
Embedded Systems Consultant,
SHDesigns home - DIY Welder