After update to latest WordPress, you probably use the new built-in Editor (Gutenberg) which format the content into separate blocks (Paragraphs, Headings, Images, Read more, etc.)
Once you get used to with it (after writing two or three posts, you probably will become addicted to Gutenberg editor … and you won’t even want to use anymore the old (Classic) editor.
When you open an pre-existing post (created before the Gutenberg update), it will be opened with the old editor. So, here is how to convert it to Gutenberg:
Click on the ‘Right-Top’ icon, then click on ‘Convert to Blocks’ item. And that’s all. Now you can update your post using Gutenberg Editor: