lbacci on "publicize opt out when editing old posts"

Hi WordPress team,
I am trying to edit some past posts to add images to content. I have Publicize, but don't want the changes to be shared in Twitter and LinkedIn as they are old posts.
When I get into a post, I click edit, and then in the Publish function I click on edit under Publicize. I see that Twitter and LinkedIn are checked, but it doesn't give me option to uncheck these boxes. What do I need to do?
Thanks for your help,
Laura