Contact the Support centre via https://support.chess.com/customer/portal/emails/new - give them the details of the other account, and they can transfer the Facebook ID from that account to this one.
I'm not sure what proof of identity they will require to prevent people from just stealing other people's Facebook connections, but they'll tell you.
Edit: Actually, it would be best if you send the message from your other account and ask them to transfer it to this one.
Edit 2: I doubt if they'll be able to merge the games from the two accounts, though.
Hi there. I have already once Connected my facebookaccount to chess.com, but that was with another chess,com-username. Now it seems impossible to connect the two things with my new chess.com-username (which is the one I have used the most). What to do?