To change your seat reservation, you need to (as yorkie stated), go to a ticket office or use their Social Media Team.
There is no form, or live chat - or anything like that, simply because it would take too much time to do such a simple request via such an extensive form. (And because nobody wants to man a live chat).
My experience three weeks ago was dire. I went through the motions for TOD to Chester from St Albans via Euston. The off-peak price was OK for me so decided that at least I would get to select seats if I booked direct with Avanti. Everything went OK up to looking at the seats that it had given me from my preferences; forward facing near to luggage racks. The latter was a preference but the latter not negotiable.
So when I checked the seat maps, I had been given two backward facing seats. I clicked on one of the reserved seats to pick it up and drop it on a suitable vacant one of a pair of forwards facing seats. When I dropped it on the seat a message came up saying that the change 'couldn't be made on the webpage', fair enough but then the next line was telling me that the original seat reserved was now
"no longer available". So now I only had one seat reserved now. I tried it again, and I lost that seat as well. Luckily I hadn't actually committe to buying the ticket so I backed out and retried. On two consecutive retries, the same happened so I gave up on Avanti. As others have found, there is no online help and nobody to speak to. I then decided to just book through Thameslink and pick up from the window on the day of travel. At least I could reason with the person giving me the tickets although TL didn't offer the ability to change seats anyway.
Flash forward to Monday 20th, whilst out and about from Chester I decided to drop in on the Avanti office in Piccadilly. The reservations on the return journey gave me two seats, one forward facing the other rearward
in a different coach! (We were travelling on a Two Together railcard)! The person there was trying to help, but her system just refused to do anything with reservations. So I gave up and had to use what knowldge I had to avoid standing from Crewe to Euston.
So what is the point of a) asking for preferences and b) offering the opportunity to make selective changes from what is offered? The airlines mange it all the time, why does Avanti not have a clue?