If you claim a reward, your loyalty card changes into a reward card (where points were subtracted from your total depending on the cost of the reward in points and the appearance of the card changes too to match the reward).
Your next action should then be to redeem the reward. You redeem the reward by showing your loyalty card to one of the business' staff and telling them you have some specific reward coming to you. The staff member will then scan your pass one more time to turn it back into a regular loyalty card and also give you your reward.
If you have a pending reward that you claimed, you CAN claim another reward as a replacement, but you will not be refunded the points of the first reward. Look at the figure on the right which is an example of a loyalty club member checking for new rewards while he already has a reward claimed (but not yet redeemed at the business). In this case, the loyalty club member has already claimed a "Free Soft Drink" and spent those points needed to actually claim that reward.
Now let's say that the loyalty club member wants a different reward that is available to him such as the "Free Veggie Pita" - he CAN claim this reward - but this will essentially replace the "Free Soft Drink" reward without refunding the points that were spent on the prior reward claim.
In fact, the loyalty club member will see a screen warning him that claiming a new reward will replace the active reward and will have to confirm before claiming the new reward. See the figure below.