Congrats on your purchase - I was going to say as SteveW ^^^, simple case of swapping the wheels over, but a basic and potentially dangerous mistake by the supplying garage
Nexen are bit of an unknown quantity on here - clearly a budget, and summer (meaning year round) tyre, but seems some are happy with them:
http://www.tyrereviews.co.uk/Tyre/Nexen/N6000.htm
Just be very wary that it will take a few hundred miles to wear off the release agent that comes new tryes and particularly at this time of year your right foot needs the lead removed to stay safe