Caveat! With the new iTunes 12, Apple has decided to change the procedure to sync your purchases from one iTunes Library to another. In order for you to successfully sync your purchases between the iTunes libraries, all that you need to do is to connect your iDevice to your MacBook or PC (for Windows users) and thereafter click on File>>>Devices>>>Sync (Name of Device). This would transfer all your purchases to the new iTunes.
Kindly find below a detailed procedure for older version of iTunes. Every other step is the same except for the above.
Sync and Erase!
Ever had to connect your iPad, iPhone or iPod into a friend’s laptop/pc and you get this warning (below) from iTunes. Worry not for there is a way around this now. The guide below would help overcome this hurdle.Image 1: The image below represents what you would see when you connect your apple device to a pc/laptop other than your main pc/laptop. Image 2: (You must have an active internet connection to proceed.) Remove the usb cable connecting the device to your pc/laptop. Click on Store Image 3: Click on “Authorise This Computer” Image 4: Enter your Apple ID (if you have one. If you don’t, you would need to sign up) and click “Authorise”. Image 5: Upon successful login, you will get a pop-up informing you of that the computer has been authorised and also let you know how many computers you have authorised. Image 6: Connect the device and you would see the warning below. Please IGNORE it by clicking on the cancel button. Image 7: Once you click CANCEL, you will see a similar screen on your iTunes. Image 8: Right click on the device (in this case “Redlaw”). Scroll down to “Transfer Purchases” and click on it. Image 9: Once you have clicked on “Transfer Purchases”, you will see a similar screen showing the progress of the transfer. Image 10: Once that is done, you will need to click on “Sync”. Image 11 Once you click on the “Sync” button, you will see a window pop-up asking you either “Erase and Sync” or Cancel. Simply click “Erase and Sync”. Image 12: You can see that the apps are greyed out. To transfer them, simply click the box beside “sync apps” Image 13: Click on the Sync Apps button Image 14: The next page should appear as displayed in the above image Image 15: Once the process is done, your apps would be restored to your device (meaning you don’t lose anything). Lastly, when you get back to your main pc, you will be asked to “sync and erase”. Just click on the button “Sync and Erase” and you can carry on with normal use.