You should change the coolant if you don't know how long it's been in and it's ability to cool the engine is as important as the oil.
Changing coolant
Mix up around 2.5 liters(0.25 US Gal) 50/50 coolant with Distilled water(Rain Water/Melted Freezer ice).I keep old Gallon plastic Oil for this cleaned out.Place a bowl under the radiator.
1: Remove the seat and tank.
2: Remove the Front Radiator guard.
3: Remove Radiator cap.
4: Remove 14mm nylon radiator drain plug with it's rubber washer.
5:Crank the engine with the kill switch activated a few times.This will help move more coolant out of the system.
6:Fill radiator(Check level with a Flashlight) to the bottom of the neck or even slightly below.
Replace the 14mm Radiator plug but only tighten gently as they are very low torque and can easily be broken.It should also have a rubber washer on it.If it doesn't have the washer you can make one out of some old Cycle inner tube and a pair of small scissors.
Bleeding system
Once you have filled the Radiator to about half an inch above the fins the cooling system needs to be bled to make sure there are no Airlocks.You do this by running the engine with the Radiator cap off and wait until the engine Temp gauge gets to the small,"Nominal" mark on the gauge and looking with the flashlight.When the you see the coolant flowing over the fins for a few times the air will have some out of the cooling system.This will also tell you that the Thermostat is working and the Water pump is doing it's job so it's a good system check
Don't forget to put the Radiator cap back on and the 14mm Drain plug in!