Boot locked computer into live Ubuntu USB

open a terminal
sudo lsblk   (to identify Linux OS partition)

sudo mkdir /mnt/myroot
sudo mount /dev/sdXY /mnt/myroot    (sdxy is obtained from line 4)

sudo chroot /mnt/myroot   (switch to root of locked linux OS with this step)
# ls to see directories
cd home 
ls    (to see user account, or accounts)
Example (my linux machine)
Man5 terry]# cd /home
[Man5 home]# ls
terry   (user account is terry)

change passwd of terry, and root if want root to have its own password
example
[Man5 home]# passwd terry
New password: 
Retype new password: 
passwd: password updated successfully
example for root
[Man5 home]# passwd root
New password: 
Retype new password: 
passwd: password updated successfully

exit and unmount from chroot

exit
sudo umount /mnt/myroot
reboot and login into locked OS with new passwords