You click edit blog post, just like you want to edit the text. then you click on the demonstration picture for the chess game you have entered. Then you click on the "insert position or game" (the tiny board icon), just like entering a new game, but you do this when the game you want to edit is chosen. This will open up the game you have entered previously. The rest is just the same as posting a new game. You just make your changes and instert it.
No, there are no ways to instert arrows.
How do you edit a game once you've inserted it into a blog? And is there any way to insert arrows/etc. when annotating games in a blog?