The simplest way is by Restarting Setup Assistant

Restart the Setup Assistant and create a new Administrator account, then use that account to reset the password to the original account.

You can restart the Setup Assistant by performing the following:

Boot into Single User Mode (hold ⌘ cmd + S at boot).

Mount the drive by typing /sbin/mount -uw / then ↩ enter.

Remove the Apple Setup Done file by typing rm /var/db/.AppleSetupDone then ↩ enter.

Reboot by typing reboot then ↩ enter.

Continue setup, making sure not to transfer data.

Setup a new administrator account. You can use this account to reset the password to the original account (through System Preferences → Users & Groups).

The new account can then be deleted once the original account has been recovered.