You need to loosen the holder (red) in which the roller is located and attach it to the new roller. This should work well with a normal kitchen knife and a moderate amount of force.

Unfortunately, we do not have any information on this as we purchase the products from different suppliers. However, products that you buy from us are guaranteed for 2 years, so if something doesn't work, you are covered. So far, this problem has only been reported by one buyer. All other products that have been sold have not yet been reported to us as defective.