Hi Dave,

By rights, the best thing to do is drain the radiator and coolant system completely so you then known the precise mix of water and antifreeze you are putting in. 

Some cars' radiators have a drain plug at the bottom, but many do not, meaning you need to take off the bottom radiator hose. If you do that, you are going to have to make sure you bleed air out of the system once it's topped up. 

I'd recommend you use a mechanic for anything other than just topping up the coolant system.