Referring to change iCloud passwrod, you can choose 'Password & Security > Change Password > Vertify Identity'. So if you've used the same password you used for your Yahoo account anywhere else, you should change those accounts. Twitter itself wasn't hacked, but it looks as if the victims reused passwords on services that were, like LinkedIn and Tumblr.This information may change as Yahoo revises this service by adding or removing features or using different service providers.
This page describes current Yahoo practices with respect to this particular service.
