You can use a serial cable and console into the unit. If you know the enable password, you should be able to wipe the unit.
If you do not know/have the credentials. Stay consoled into the unit, power it off and back on.
Pay attention to the screen. It will give you an opportunity to press ESC key, to go into bootstrap. You usually only have about 5 seconds to do so.
From there, you type bypass password and press return. This temporarily disables the need for passwords when consoled in.
They type boot and press return. This boot the unit with the existing configuration, but does not require a password at the console.
Then you can go into global configuration and setup new users and passwords, or wipe out the startup-config and reboot it.
I like just changing the password if you need to see how the unit is configured.