My iPhone is in sync with my sister's Macbook pro and i just recently got the new macbook and would like to sync my phone with my own computer but it cannot sync because it is already in sync with the other one, how can i 'un-sync' from the other computer without resetting anything?
iPhone 5s, iOS 8.3, Using MacBooks
Connect your iPhone to your sister's Macbook Pro.
Launch iTunes. From the iTunes menu bar click Store > Deauthorize This Computer
You should be prompted to enter your Apple ID and password to proceed.
More help here > Deauthorize your computer using iTunes
An iPhone can only sync with one library at a time. If you want to start syncing with your own computer you can either, 1) Let the phone be erased and sync with your library, or, 2) Copy your sister's library to your computer so iTunes is still syncing with the same library it was before. You do the second by copying the entire iTunes folder to your computer, start iTunes while holding down the option key, and selecting the moved library folder.
How can i disconnect my phone from sycning with another computer?
