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When updating an app iOS prompts for password to a completely different, not yours AppStore account.
This is because this app was installed under another Appstore account, most probably, by the previous owner of you iPad or iPhone. You have to delete all those apps by holding their icon on the Springboard until it rumbles, and then clicking the cross symbol. Uninstall all such apps, and then go to Appstore under your account and download them again.