The ticket has no value for change or refund after your first train departs...I have a feeling this journey was very early in the morning, wasn't it? In that case you'll have trouble getting through to anyone in order to change it.
The full Ts & Cs w.r.t. changing advance tickets are here:
Changing the time or date of travel
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Changes to time or date of travel must be arranged before departure of the first reserved train printed on the ticket, after which the ticket has no value and a new one must be purchased. You will need to present the ticket(s) and reservation(s) when you request a change.
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Changes to tickets cannot be made on-board the train. If you board a train without a ticket and reservation for that service, a new ticket must be purchased. Depending on the Train Company you are travelling with, you may be liable to a Penalty Fare if you board the train with an invalid ticket.
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The origin, destination and Train Company or route shown on the ticket(s) must remain the same.
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The difference between the price paid and cost of the next suitable fare for your journey is payable, plus a ^10 administration fee per person, per single ticket for each change to a journey. If you change to a train on which a cheaper fare is available, the difference will not be refunded.
Good luck...