You can upgrade your game via using the Premium code of a boxed game (like the Premium edition or Premium as a boxed version) or if you buy Premium from Origin or your specific console store. Please keep in mind that you don't need the boxed Premium edition if you already own the basic Battlefield 4 game. Getting Premium from your specific store should be the easiest way to upgrade your account.
After you bought Premium, you can downlaod the Premium content from your Origin game detal page or from your console store. Please keep in mind that you need to get the Premium download first before downloading the expansions.
Cheers
Basti
