A lot of people aren’t fond of the idea of using liquids on their cardboard card backs to remove a sticker. If you are one of those people, This is a tip for you!
Instead of liquids, use a low setting hair dryer. Blow the heat over the sticker for a little while (until the glue softens), and then gently and slowly pull the sticker up with tweezers. Continue to use the heat while lifting the sticker. Patience is critical with this one, as it can easily rip the cardback if done fast.
NOTE: Keep the heat AWAY from the cards bubble, because it can warp the bubble badly. Also note, this method may leave glue residue on the cardback. Also, I have heard that this only works if the sticker still has some tackiness to it. If the sticker is dry and you overheat the card, it may damage the cardback.