How to Work with Versions in Google Docs
After you create a new document, every change or edit you make is recorded. The record of these changes is called the version history.
The Version history pane appears at the right showing all previous versions that have been autosaved.
You’ll see a preview of that version at the left.
This allows you to see the changes that were made to a version.
Any changes that were made are highlighted in green.
You can name a version to make it easier to find.
The version is named.
Now, selecting a version in this pane doesn't actually bring this version back; it just displays a preview. You could still click the back arrow and return to the current version.
The older version is restored to the active version and you can continue making edits.