Create a linux live USB drive. Shutdown the computer that yuo forgot the password to. Plug in the live USB and boot up, interrupting the sequence to get to the BIOS. Once the live USB boots, mount the computer's drive by opening the file manager and clicking on the drive. Right click on the folder where it was mounted and click "open in terminal." Run the following command to enter a chroot environment as if you were logged into the computer you forgot the password to:
In the new environment, change the password of the user you want:
You can also change the root password:
Then reboot the computer and log in with your new password!