if i have used someone elses apple id to download an app, will it erase when i update my phone on itunes? i used my friends apple id to download a music app that she had downloaded as i could not find it on my apple id, will it erase if i update my phone on my iTunes off my laptop?
The app will not be erased and you can use it. But as Tetsujin said, you cannot update the app to the latest version without your friend's app Apple Id.