There's "change reservation" available from the menu bar in the middle of the screen, next to the flight search options - select UK as your country site. Unfortunately I don't think you can change seat assignments through this functionality, but give it a try anyway, maybe they've added that in the recent times - I don't have any suitable reservations at the mo to try it. Oh and this only works if you booked your flight from Finnair and not through other airlines or TAs (both physical and web).

If it won't work, then contact either your travel agent if you used one or the Finnair call centre directly (probably completely overstressed at the moment due to the pilot strike).