Go to manage my booking:http://www.thaiairways.com/en/Manage..._Booking.page?
and pull up your reservation.

Scroll down and click 'Additional services' as if you wish to change seats.

either click the '+' sign on the right of the words 'Your Booking', or scroll down the page to see the yellow 'continue button. Just under this you can click on 'Purchase Conditions'

Cancel out of everything once you read the details.