I just again suffered the same fate, and booked a LH flight while logged in. I credit all my miles to UA, so needed to change but didn’t have a button.
Luckly, Austrian’s website is much easier to use, and lets you edit the FFN easily:
https://book.austrian.com/app/fb.fly...ion=pnr_search
To verify the change, mail yourself the invoice from Swiss’ website (oh, Lufthansa, when will you fix your IT problems...) since of course LH is broken:
https://www.swiss.com/ch/en/Login