The accumulator contains a precharge of Nitrogen gas to a certain pressure (80bar/1160psi) and over time this precharge dissipates and the accumulator assistance to boost the hydraulic pressure reduces meaning the boost pump has to run more often, this extended duty of the pump can lead it to fail prematurely.
The way to change it is to ensure ignition is OFF, depress the brake 30 times, wait 60 seconds then do 4 more full pedal depressions and this will make sure the brake system is depressurised fully.
Then spin off the accumulator catching any fluid loss, then replace with new, NEVER PIERCE THE ACCUMULATOR. Then Bleed to braking system TO THE LETTER as in RAVE.