How to change password if you forgot yours

See If you forgot your Apple ID password https://support.apple.com/en-us/HT201487

You also can use Having trouble signing in/iForgot https://iforgot.apple.com/password/verify/appleid 

And you will need to see If you forgot the answers to your Apple ID security questions https://support.apple.com/en-us/HT201485

Before trying to answer your security questions is very important that you think about your questions, including your birth date, and their answers carefully and try to recall all details you can. If you don't answer them correctly enough, then you easily can lose control of your account.

Note that If you're using two-factor authentication and can’t recover your password, you usually will need to enter account recovery. See Recover your Apple ID when you can’t reset your password  https://support.apple.com/en-us/HT204921