How do I upgrade my duck?

How do I upgrade my duck? - A Woman Protesting with Messages on Band Aids

So the new TF2 EOTL update features a thing called a duck, which you can equip. It says you can find ducks from fallen enemies and use them to upgrade your duck.

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Now i"m a little confused because in the store there is a Duck Token, which also appears to level up the Duck. So what's the difference between just killing enemies to get ducks as opposed to getting Duck Tokens?

How do I level up, or upgrade my duck?






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Sources: Stack Exchange - This article follows the attribution requirements of Stack Exchange and is licensed under CC BY-SA 3.0.

Images: ShotPot, Nataliya Vaitkevich, Brett Jordan, Brett Jordan