Once you schedule a post, you can edit it. To do that:
Open your calendar section from the menu bar on the left.
Choose the accounts in the upper left corner.
Posts scheduled for previously chosen accounts will be displayed.
Choose the one that you would like to edit and click on the three dots button in the upper right corner of the post.
The drop-down list will be shown. Choose the Edit button by clicking on it.
Now, the post you would like to edit will be displayed in the creating mode again.