Take a turkey baster, suck out all the old fluid to just above whatever lines connect to the resivoir. Fill it up. Drive around a mile or two. Cool the car down if necessary. Repeat 2 or 3 times to make sure you change the old fluild pretty good.
Don't let the fluid get beneath the lines or you'll introduce air into the system -- not good. If you do that, you'll have to bleed the system, and you'd better let a mechanic do that.
Throw the turkey baster away, or at least keep it out of the kitchen.