Go to global configuration mode by below command
ASA5505# config terminal
Now give below command to make firewall at default setting
ASA5505(config)# config factory-default
If you want to get back to the prompt that looks like: ‘ciscoasa(config)#’
Then you have to follow third step
ASA5505(config)# reload save-config noconfirm
Now make sure that the outside line is plugged into port ethernet0, and your pc is plugged into any of the ports 1-7.
The Cisco ASA has been reset to factory settings. DHCP is enabled on the cisco device, and it’s internal IP address is now 192.168.1.1!