I bought a tin of ready-made chocolate icing from the super market. But it's very dense and viscous while I prefer the more airy kind of icing the cakes from the bakery have. Can I do something to turn my icing into something like that?
Just to summarise the comments into an answer:
To lighten the icing, you can add air to it by whipping, either by hand or with an electric mixer. It will probably be easier to do that if you loosen it a little with some milk or cream.