Update your Yahoo Mail password on Android Mail
Android mobile devices come with a pre-installed email app where you save your Yahoo username and password. When you change your Yahoo account password you also need to update your old password stored on your phone to match your new password.
Based on your version of Android you may have different ways to update the old password within your mail app settings. Please check with your device’s manufacturer for specific details.
If you're still unable to see new mail, remove and re-add Yahoo Mail from the device.