There's lots of posted information concerning this procedure.
All you need to do is pump the brake pedal (with key off) about 12-15 times to de-pressurize the accumulator. The pedal will be very hard after all the pressure is relieved. Put a rag under the old accumulator to catch any drips then screw it out. Lubricate the o-ring of the new accumulator with brake fluid and screw it in, torque to about 15-20 lbs. After the new one is installed, pump it down (de-pressurize) a couple times to insure all the air is out of it.
If your brake fluid is old and dirty, or been in the car for a long time, you should replace it with fresh fluid before changing out the accumulators.
