Tested on an HP 712/100 workstation.

1. Press escape at the boot banner.
2. Use 'search' at the prompt to get a list of bootable devices.
3. Boot the correct device with parameter 'isl':

   boot scsi.6.0 isl

4. Use the following command on the ISL prompt to boot the kernel into single-user mode:

   hpux -is

The system will be booted to a superuser prompt.

Source:UnixHub.com - HP-UX boot guide

1. Boot into single-user mode.
2. Back-up the current passwd file:

   mv /etc/passwd /etc/passwd.orig
   cp /etc/passwd.orig /etc/passwd

3. Open /etc/passwd in vi.
4. Remove the text between the first and second colon on the line that begins with 'root'. Example after edit:

   root::0:3::/sbin/sh

5. Reboot the system.
6. Login should now be possible for root by just entering the username.
7. Set a new password for the root account with 'passwd'.