An upgradable offer is an offer where the Cash Back increases after gifting it to your friends on Facebook or Twitter.
Wondering how to gift an offer to your friends? Just look at your offer list for an offer with the word upgrade in the second line, tap on the offer to go to the offer details page. Once you're there, tap "Gift This" to post the offer on Facebook or Twitter so your friends can see it.
Once you've gifted the offer, the Cash Back of your offer will increase, and your friends will have a chance to earn Cash Back too!