I don't think you can, I'd have to do it all over again, and it took me a while the first time, stubborn_donkey very helpfully put up the right one anyway.

Interesting.

271, I read.

You click on FEN/PGN and copy the content to the clipboard then paste in notepad so that you have a copy.

Then you can edit the post, add a new diagram, put in the PGN position from the clipboard or copy from notepad and then you can edit the new diagram, delete the old one and submit the changed post. Make sure you keep the backup in notepad until you have finished the action.

not the longest ever, but a fun mate in 24, that you can actually do in your head ;D

Yeah, pretty basic stuff...mate in 290 :) 

haha