Depends what type of ticket you have purchased. You can usually refund Off Peak or Anytime tickets for a £10 admin feee.....but I suspect you have an Advance ticket, which is non-refundable I'm afraid. Virgin will allow you to AMEND an Advance ticket for date or time, travelling between the same two stations, for a £10 admin fee.
So if you've got an Advance ticket, it's probably best to go about amending the date/time.