1. go to there site and "contact us"
2. complain that you had a problem with their drink (bad taste, not carbonated like normally, smells strange)
3. wait a day or two after which they will contact you via email and ask you where did you bought it and to provide them a product code on the bottom of cans
4. tell them that you got a pack as gift or that you won on some contest hosted by some small local store ( I said this)
5. they will respond something like this:

I provided my real phone number when I contacted them so day after this mail, some girl called me and said that she is sorry about a drink and to ask me when I will be home so she can send the girls to give me a replacement pack. I wanted to use a drop, so I could do this more then once, but unfortunately they insisted to give me directly so I said not a problem then. Anyway after 3-4 hours red bull car was at my door and 2 very nice girls gave me my 24-pack (one girl goes to my college so I got her number too haha). 

As I wrote they contacted me on phone so I don't have a mail in which is written that I will get a replacement, but this picture will probably be adequate proof.