Sure! Go to Publishing and in Calendar select the post you want to edit - remember, all scheduled posts are colored in light blue, for better identification.
The dark blue highlight shows you all the linked posts - those that were scheduled in one go across different profiles and dates
Then click on the post and edit it. Easy :)
Note: If your post is linked to another post, that is, if you crafted similar content to go out on multiple profiles, including Twitter, then opening the edit modal will open up the composer, the same way as during creation.