Welcome to Flyertalk.

To use miles to upgrade, you first have to pay to upgrade your economy ticket to Y B or L class - that can often be VERY expensive. See the Virgin website here.

Having said that, depending on how busy your flight is, and how much they NEED to shift people forward, it has been known for the rules to be flexible at the airport on your day of travel.

If you have booked direct with Virgin, they will tell you how much it will cost you. If you have booked through an agent, you will probably have to sort out the upgrade through them.

Nick