This sort of change to a journey is allowed on most types of train ticket in the UK. Unfortunately, an advance ticket is an exception, and does not allow any changes at all; you have to board and leave at the stations specified, not in between. (Advance tickets are notorious for being particularly inflexible.)
However, you say that the journey is still some time in the future. It is often possible to request an exchange for your Advance ticket, updating the details to a different set of details; with cardboard tickets this would be done via handing in the tickets at the ticket office of any staffed station, but the process may be different or nonexistent for mobile tickets. (With mobile tickets in particular, the terms and conditions say "However, please note that if you choose (where offered) to print your tickets at home or download them to your mobile device, different rules apply and you may not be able to subsequently change your ticket."; it's nonetheless worth checking to see if a change is possible, because "may not be possible" doesn't imply "is never possible".) Note that there is a fee for the change in journey, normally £10 plus the difference in price; as such, it's a good idea to look into the possibility of an exchange as soon as possible, as the price is normally lower the earlier you book the ticket.
It's quite possible that (as you're changing to a shorter journey) the difference in price is less than minus £10, so (assuming that the exchange is technically possible) it may be possible to convince the ticket office staff to exchange the ticket for free.