‘An online retailer sent me the wrong Lego set. Should they be able to exchange it for free?’

Q I bought a Lego set online as a Christmas present for an eight-year-old. Something made me open the package – and they’ve sent me the wrong set. I ‘d ordered a very specific Lego set and I don’t want to give her this one instead.

What are my rights here? Do I have to post it back?.