Firstly we need to enable automatic download on the computer of yours for iTunes store, iBookstore, or App Store. Follow these steps:
1. Launch iTunes, and authorize your PC using your Apple ID, by going to Store > Authorize This Computer. (Skip this if your PC is already authorized).
2. And now it's time to set enable automatic download.
On Mac computers: Select iTunes > Preferences then press the Store tab.
On Windows computers: Select Edit > Preferences then press the Store tab.
3. Now, select or check which items you like to set for Automatic Downloads, whether its Music, Apps, or Books under "Automatic Downloads." Or check all of them!
Remember this tips applies only for new purchases made after the automatic download enable.