Thanks for the question, and welcome to Apple Support Communities.
The following article provides an answer to your question:
When a pre-order item becomes available, you will receive an email letting you know your pre-order is available for download.
If you've enabled iTunes to automatically download prepurchased content, then your pre-order content may already be downloaded to your library. If not, simply click the download link provided within your email notification, and your pre-order content will begin to download.
To have prepurchased content automatically download in the future, make sure Automatic Downloads are setup on your device: