You are not legally entitled to change an item because you have changed your mind. However, if there is a fault with it, you can ask for a refund. Speak to the Manager nicely and he may use his discretion to extend the 16 day no quibble refund policy for you.
In my experience Argos are very good at letting you change unwanted items so long as they are in resalable condition and you have the receipt - even after 16 days. I expect this is at the descretion of the manager, though. Of course if the item is faulty, you are entitled to a refund by law.