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I have 2 Apple Ids on my phone I purchased an App on 1 id but when I tried to upgrade it used the other id and the App wouldn't accept it as different account name. Asking me to download the App for this id. but can;t because the App is already installed on my phone.
you have to use the same ID you used to purchase an App to update it.
Normally when you go to the App Store and tap update for an App your iPhone will prompt you for the correct ID needed to update that particular App, but if this is not happening, then you should go to Settings, Store, sign out of the other Apple ID and log in with the one you need to update the App that is already installed on your iPhone..

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