I believe we have met before.
So you just wish to change your Yahoo password.
That is very simple if you have registered the secret questions and remember the answers. The secret questions will give you immediate access to you account and from there you can change your password.
If you don't have a secret question, you must an alternate email address or phone number which you registered when you created the account. A reset code would then be sent to you via the alternate email.
If you don't have one of the above, you may consider your account lost for good. If you do, let me know and I shall provide further guidance.