This pack will upgrade you to the Horizon Deluxe Edition which will provide you with the following:

Remastered Game Units and Characters

Experience the game with all combat ships and race characters remastered to look sharper and more detailed. Ships resolution was increased by 200% and character animations by 150% of the original.

Digital Original Soundtrack

The Horizon Original Soundtrack delivers over 30 minutes of original music composed by Dustin Crenna, including bonus tracks and alternate versions not included in the game.

Wallpapers

Enjoy the inspiring Digital Paintings of Horizon in Ultra HD 4K+ format. Also included are CGI art from the trailers.
Bonus items: The Box Art and the Manual (in PDF format) for fans who like to print and make their own physical box copies.

The Soundtrack, the wallpapers and all other DLC content can be found under the Horizon Installation Folder. To navigate there "Right-click" on Horizon under your Steam Library then select "Manage", "Browse Local Files".