You have several options. The easiest would be to check in at the airport and get the agent to put in the details there and then. Or check in online, keep the details as they are and then change them on arrival at the airport as mentioned on their website. You can contact them via the social media outlets which are usually quite quick to reply. Or as mentioned try another browser as the squeezy "same legroom as British Airways" Jet website may work better on other browsers. You could also try the app, I know you can add API data but not 100% sure if you can amend the data.