*The below testing/snippets are from a CSG770. The Grub outputs on general White boxes may not be exactly the same as shown below.

Step 1:

Reboot the appliance, and then keep pressing “Left-Shift” and  “Esc” back to back until you enter the Grub mode

Step 2:

Select Advanced options for Ubuntu

Step 3:

Select the first option as shown in the below snippet and press “e”

Step 4:

Add “single rw init=/bin/bash” in the line where you see linux. Then press ctrl-x or F10

Note: These changes are not persistent. Any change to the kernel boot options made this way will only affect the next boot and only if you start that boot by pressing either Ctrl+X or F10 while still in GRUB edit mode.

Step 5: 

Press Enter to for maintenance mode

Update the password using the “sudo passwd admin” command

In addition, if the device has external authentication (TACACS/RADIUS), you must edit two additional files. You must delete the first two lines from /etc/pam.d/sshd and /etc/pam.d/login

Step 6: 

Power recycle the Appliance.