Figure out the account short name. Same as the home folder name.
Boot into single user mode (press Command-S at power on -- At startup Chime)
Launch core system services by typing
Change the password for the account's short name, with (No Angle brackets)
Enter the new password, press return, type it again, press return, it should say something like "password changed"; it won't echo what you're typing, but it will be working, just trust it.
Restart the computer by typing
You should be able to login with the new password.