If your booking reference is a 6-letter combination, good news, it is free of charge to change your seat for a Eurostar ticket.
To do so, visit the booking section of Eurostar (https://managebooking.eurostar.com/uk-en).
Once you’ve entered your booking reference and name to access your booking, you can then proceed to change the seat by simply selecting “Change seat”.
Unfortunately, if your booking reference is composed of a mix of letters and numbers, it won’t be possible to change your seat before travelling. Seat changes are not possible after the booking has been finalised due to the parameters of the reservation system. So once you have made your booking and/or paid for your ticket, your seating is final.
Unlike airlines, the seating arrangements on trains are quite strict and it is not as simple to move passengers around once a booking has been made.
Your chances of getting the seats that you want are much higher if you book early. Eurostar tickets are available 6 months before travel.