Plug in your serial console,
reboot the ASA
press 'ESC' when its booting
# Now get into configuration management
rommon #0> confreg
# Record this next line we'll need it later.
Current Configuration Register: 0x00000001
Configuration Summary:
boot default image from Flash
Do you wish to change this configuration? y/n [n]: Y
enable boot to ROMMON prompt? y/n [n]:n
enable TFTP netboot? y/n [n]:n
enable Flash boot? y/n [n]:n
select specific Flash image index? y/n [n]:n
disable system configuration? y/n [n]: Y
go to ROMMON prompt if netboot fails? y/n [n]:n
enable passing NVRAM file specs in auto-boot mode? y/n [n]:n
disable display of BREAK or ESC key prompt during auto-boot? y/n [n]:n
Current Configuration Register: 0x00000040
Configuration Summary:
boot ROMMON
ignore system configuration
Update Config Register (0x40) in NVRAM...
#now lets load the clean settings
rommon #2> boot
hostname# copy startup-config running-config
hostname# configure terminal
hostname(config)# password password
hostname(config)# enable password cisco123
hostname(config)# username admin password cisco123
# Remember that line I asked you to record
hostname(config)# config-register 0x00000001
hostname(config)# copy running-config startup-config
# Reset to factory settings
config term
config factory-default
# Hit spacebar when the ‘more’ thing happens.
(Optional) reload save-config noconfirm