This method is only if you forgot your current user password! Changing your current password should be done through the desktop environment.
Warning
Follow this guide at your own discretion because you can break your system attempting any of this.
Reboot your device.
Press Esc on the keyboard to reach the GRUB boot menu.
a. If you press Esc too many times, you may end up at a grub> prompt.
b. Return to the boot menu by typing exit and pressing Enter
Edit the last deployment by presssing E on your keyboard.
Edit the GRUB prompt and append init=/bin/bash to the line beginning with linux.