You'll have to use Grub boot rescue mode to gain root privileges. Select
Grub's entry "Advanced" at boot.
With full disk encryption, edit /etc/default/grub --> comment out (prepend
with #) line 'GRUB_HIDDEN_TIMEOUT=0'. After changes to Grub, reload grub
configuration 'sudo update-grub'.
Then select drop to prompt and proceed to change your account's password. If
the drive is mounted read only, permissions must be changed and partitions
remounted.
# passwd
https://askubuntu.com/questions/16042/how-to-get-to-the-grub-menu-at-boot-time
https://askubuntu.com/questions/826282/full-disk-encryption-how-to-boot-in-as-runlevel-1
https://wiki.ubuntu.com/RecoveryMode
