Open the app and make sure you're signed in (top right)
Go to 'My Tickets'.
Select the journey you've pre-purchased that you wish to change the date/time of.
Tap "Change date or time of travel" if this is not available your ticket is not valid for change of journey
Select a new date and time for your journey's outward and return trains
Select your preferred new tickets for both outwards and return
Checkout as normal, only paying any outstanding balance if you've selected a more expensive ticket
Once you have confirmed your changes, your journey's date and time of travel will be updated