For the Web Browser version
For Titles and descriptions, you may be able to undo changes by selecting the relevant Text field and pressing Ctrl+Z (hold the Control key and press the Z key).
It is not possible to undo Step additions or deletions, or image and video additions, deletions, or replacements one at a time.
If you have accidentally edited or deleted something, please make use of "Discard unpublished changes" or "Backup".
▼Discard unpublished changes to the Manuals (Web Browser)
https://teachme.jp/8/manuals/33088474
▼Restore manual from backup on the manual editing screen (Web Browser)
https://teachme.jp/8/manuals/9917804
For the mobile app
It is not possible to undo edits one at a time.
You will need to restore a Backup using the Web Browser version, or make corrections manually.