You can spend points under the 'ways to redeem' section of your loyalty panel. Here you will find the amount of points that are required to claim/get a reward/discount.
Once you have enough available points, you can simply select 'Redeem' - again within the panel; click 'Apply code' for the discount to be automatically applied at checkout. Once you redeem a reward, it will appear under 'Your Rewards'. It's worth noting that only one reward is allowed per order.
Note: If you need to cancel or undo a points redemption, you will have to contact the store's customer support.