For many HW/SW-combinations this is documented in"Password Recovery Procedure for the Cisco Aironet Equipment":
http://www.cisco.com/en/US/partner/products/hw/wireless/ps430/products_password_recovery09186a00800949d0.shtml
In most cases this will be a factory reset. The previous configuration,
including username/password, will be lost.
For VxWorks versions 11.07 and newer it's as simple as a ":resetall" on the
serial console in the first 2 minutes after a reboot.