How To Reset Password on Linux Mint and Ubuntu
The easiest method of resetting a user’s account password in Linux is to use the passwd command. To do it on Linux Mint, Ubuntu or any Linux distribution that uses sudo, start a shell terminal and type the following command:
sudo passwd
You will, of course, be prompted to authenticate with your current password before you are given the opportunity of changing it. On a distribution that does not use sudo, call the passwd command without sudo.
This screenshot shows the steps involved.

If you do not want to use a shell terminal, use the graphical user management application that comes with your distribution. That will give you a point-and-click interface to reset the password.