What do you mean by an exchange?

Bubbles 101: Carbonating cylinders create the bubbles. After some time, the carbonating cylinder runs out of CO2.   All you have to do is buy a new cylinder and we will deliver this to your door or a convenient Collect+ location. All you need to do is use the packaging from your new cylinder, and the postage label we will put into your parcel, and drop off at a Collect+ store. Within 5 days of us getting the empty cylinder back, we will give you a £10 refund against what you have paid (if you bought the cylinder at sodastream.co.uk)  We offer an exchange, in which you get a new carbonating cylinder in exchange for your empty one.