To edit a scheduled post, first you will need to find the post in either the Content Calendar or the Outbox. You can find both of these under the Marketing module:
These two screens display your published and scheduled messages, the first in a Day/Week/Month view and the second in a list format.
Once you have found the post you want to edit, hover over the 3-dot icon and click on the pencil option:
Clicking this will bring up the compose tab, where you will be able to edit the Campaign, Text, Media, Targeting and Schedule of the post (where relevant and necessary).
Note: You will be unable to change the social accounts that the post has been scheduled for, as this aspect gets locked into the relevant network queue when it is initially scheduled. You will also be unable to edit a message if it is about to be posted, as it will have already been locked into the publishing queue.
Once you have made the changes, click on the “Update X schedules”: