Problem with updating whatsapp?

When I downloaded the first ever app on my iphone (whatsapp) my friend used her apple ID and now when I try to update it it asks for her password instead of mine which I don't know:/ what can I do? I don't want to delete the app and reinstall because I don't wanna lose my chats help

Added (1). She's on holiday right now and I'm trying to update to save my chats for when I get my new phone tomorrow:/

Ask her what her password is.

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