Using anyone's Apple iPhone, or iPad, open the app Find my iPhone.
Click Sign Out at the top right and then input the Apple ID you want to reset the password on.
Next click forgot my password.
A text message will be sent to the number on file for that Apple ID, or you can choose to be called.
After you input the 6 digit code, you will then be prompted to input the phone's unlock PIN.
Last but not least you will create a new password 8 digits or more, including a number.