First, interrupt the boot to go into ROMMON mode. Here is what I did when I stopped the boot process below. Notice that the original boot config register ends in "01". I dont want that next time it boots. I want it to be "41". You can see that after I changed the config register, I rebooted the ASA.
rommon #0> confreg
Current Configuration Register: 0x00000001
boot default image from Flash
Do you wish to change this configuration? y/n [n]: n
rommon #1> confreg 0x41
Update Config Register (0x41) in NVRAM...
Next, I booted the ASA normally. However, it acts like its never been configured. Notice below, I get the original config back in place by doing the copy start run. Then, I change my enable password and my username password. Then I change the config register back to the way it was before ("01").
ciscoasa# copy startup-config running-config
Destination filename [running-config]?
..INFO: outside interface address added to PAT pool
INFO: Global 188.8.131.52 will be Port Address Translated
Cryptochecksum (unchanged): 04a475a6 81a5f851 a6e7af85 85317660
10169 bytes copied in 4.180 secs (2542 bytes/sec)
ASA# config t
ASA(config)# enable password newpassword
ASA(config)# username shane pass shane pri 15
ASA(config)# config-register 0x01
Before I reload the ASA, notice I do a show version. Notice the highlighted piece of it, just FYI.
ASA# show ver
Cisco Adaptive Security Appliance Software Version 8.0(4)
Device Manager Version 7.1(3)
Compiled on Thu 07-Aug-08 20:53 by builders
System image file is "disk0:/asa804-k8.bin"
Config file at boot was "startup-config"
ASA up 1 min 34 secs
Hardware: ASA5510, 256 MB RAM, CPU Pentium 4 Celeron 1600 MHz
Internal ATA Compact Flash, 256MB
BIOS Flash M50FW080 @ 0xffe00000, 1024KB
Encryption hardware device : Cisco ASA-55x0 on-board accelerator (revision 0x0)
...edited for brevity...
This platform has a Base license.
Serial Number: JMXXXXXX
Configuration register is 0x41 (will be 0x1 at next reload)
Configuration last modified by enable_15 at 11:28:17.629 UTC Wed May 14 2014