pct enter
passwd
exit
Method 1
Boot into your bootloader (mostly grub or systemd-boot, select your preferred kernel but do not press enter.
Press e to go into edit mode.
Depending on the boot-loader go to the kernel-commandline entry:
For grub: scroll down to the kernel line you will boot from, it starts with linux /vmlinuz-…
For systemd-boot the kernel commandline is directly editable
Scroll to the end of that line, optionally remove the quiet in the line and append init=/bin/bash
For grub press Ctrl X to boot, for systemd-boot simply press Enter
In the resulting shell:
# Remount / as Read/Write
mount -o remount,rw /
# Change the root account password with
passwd
# Change any other account password with
passwd username
# type new password, confirm and hit enter and then reboot.