How do I replace the battery?

Dear Chrissi! FitBit Flex is made as a "one time use/throw away" device. Perhaps the 25% off is a way of getting repeat buyers as the batteries fade...? The device is not made to be opened and the only way of "repair" is a way that is seldom successful. The device has to be opened with a saw! If you want a new FitBit with the same lifespan than I suggest you take advantage of the 25% discount..... Otherwise I suggest you check out other options where the batteries are more "user friendly". If you want to see more details of how to make an effort..... Search "FitBit Flex" here on "IFIXIT" and choose "FitBit Flex Tear down". Good Luck!