Like Pages, Posts consist of editable Blocks, so when you edit a Post, you're really editing that Post's Blocks.
Steps to Editing a Post
Hover over the Dashboard button in the Toolbar and click Posts.
The Posts panel will appear.
Click on the Post you want to edit.
The Edit Post panel will open.
To edit a specific Block in a Post, click on that Block and press Edit. See Editing existing Blocks for more on editing block content (The linked article is about Pages, but since Posts use the same Blocks, it's relevant).