i have '04 saab 9-3 arc show me low coolant, refill. so what i have to do refill or first drain old one?

if the coolant looks fresh, would not bother to drain, but first find out from where the coolant went, if it was burned through a leaking head gasket, it's not going to heal itself, but if it is a leaking hose, hoses will get sloppy and loose with time, in which case you're going to want to drain the coolant at the lower petcock to replace the lower hose and while you're at it the upper hose and the cooling/heating hoses too, okay yes, drain the coolant, but use that down time to affect the rubber replacement~

if its gone into the engine head gasket replacement easy way to find out remove oil filler cap look on the underside of it if its white like mayonnaise water contaminated oil hopefully for you that its not the case and just a leaky radiator or hose