How to change Yahoo phone number without signing in?

To change your phone number on Yahoo without signing in, you will need to regain access to your account first. If you cannot sign in, use the Yahoo Sign-in Helper. Enter your email address and, if prompted, try alternative recovery methods such as using an alternate email or answering security questions. Once you regain access to your account , sign in and navigate to the Account Security section under Account Info. From there, you can update your phone number. If recovery options fail, contacting Yahoo Support for further assistance is your best alternative.

Source: https://community.thermaltake.com/index.php?/topic/452349-how-to-change-yahoo-phone-number-without-signing-in/