We've an article that provides the following details on downloading your pre-orders when they become available.
Download a pre-ordered item
When a pre-ordered item becomes available, you will receive an email letting you know that you can download your pre-order.
If you enabled the option to automatically download prepurchased content in iTunes, then your pre-ordered content may already be in your library. If not, click the download link provided in your email notification, and your pre-ordered content will begin to download. You can also use the Check for Available Downloads feature in iTunes on your computer (choose Store > Check for Available Downloads) to begin downloading your pre-order.