This depends on the terms and conditions of the ticket you have purchased.
If your ticket is an Anytime, Off-Peak or Super Off-Peak fare and you wish to change we advise you to cancel and obtain a refund and re-book. If you are travelling on the same day but need a ticket that allows an earlier or more restricted time, you can pay the difference at a ticket office to make your existing ticket valid for that journey. This is done at a ticket office. It does not matter where the original ticket was bought or whether it is paper or digitally purchased.
If your ticket is a First or Standard Advance (for travel on a specific booked train), your ticket can be amended prior to the booked time of departure, subject to an amendment fee of £10 per ticket plus any difference in fare that there may be to travel at the new time.
Please contact the original point of sale, or ask at any station ticket office, for information on how to amend a First or Standard Advance ticket.