User`s guide

you can continue. To delete the password, type the current password
immediately followed by a backslash (\) and press the Enter key.
NOTE: If you don't have access to the power on password, you must
disable the power on password by removing the real time clock
battery. Refer to section 3.11, "Real Time Clock Battery," for
removal procedures. Once the real time clock battery is removed,
short the real time clock battery terminals in the processor board
by touching the battery terminals with two ends of a wire. Replace
all components, and resume POST.