As per
https://www.nationalrail.co.uk/ticke...d-for-tickets/
Changes to tickets can be made at most National Rail ticket offices before you travel. Alternatively, you can contact the online retailer that you bought your ticket from.
However... I've overheard quite a few ticket office staff telling people that they need to contact the website they bought the ticket from, so I'm not sure how much luck you'll have in-person
In most cases I'd say contact the your issuer, however, getting in touch with Uber is rarely much fun... Would probably still suggest trying Uber first though!