NOTE: Only document owners and users who have been granted editor permissions to a document can restore document versions.
Click the version history information next to the document's name.
Find the version you'd like to restore - click to preview, and select Restore.
Confirm by clicking Restore.
A confirmation banner will pop up once restoration is completed.
WARNING: Any changes made to this document will be automatically synced back to Google Drive.
On the free, starter, old professional and old business plans:
Only the latest 5 versions of documents are restorable.
Older versions are deleted after 7 days.
On the pro, business and enterprise plans:
The latest 50 versions of documents are restorable.
Older versions may be deleted after 30 days.
Users can choose to keep specific versions of documents by selecting 'Keep forever' in Google Drive.
Keep versions of files in Google Drive by following these steps:
Right-click on the file.
Head down to File information -> Manage versions.
Locate the version you want to keep, and then click the menu button.
Then, select 'Keep forever'.
Reference: For more information, visit this article from Google.