Q: Change iphone Apple ID & PW

Do you mean you went to your existing Apple account and changed the Apple ID email address to a new address? If so, then you can sign out using your current password.

If you mean that you created a brand new Apple account, then you will have to use the password that you set for that account. If you don't remember the password go to: Apple - My Apple ID and click on Forgot Password. If you don't remember the answers to your Security Questions, you can contact Apple Support to get a reset link sent to you: ACCOUNT SECURITY SUPPORT

Here is an article about where to change your Apple ID when you have updated or replaced it: What to do after you change your Apple ID email address or password - Apple Support

Best of luck,

GB