No, you cannot.
Please create a new folder of the desired type and move the manuals from the existing folder to the new one.
▼Create folder (Web Browser)
https://teachme.jp/8/manuals/4929440
▼Change manual folder (Web Browser)
https://teachme.jp/8/manuals/4929397
Related: What is the difference between internal and public folder
https://help.teachme.jp/hc/en-us/articles/900007185943
If you no longer want to share manuals that are already created in a Public Folder, you can set the folder to "Unrelease".
Setting a folder to "Unrelease"
By selecting "Unrelease" for "Outside Release" in the folder's "Release Settings", users who are not logged in will no longer be able to view the folder or manuals within it.
▼Screen that appears when an external user clicks on a folder URL while it's set to "Unrelease"
Note that users who meet all of the following conditions can still view the Public Folder even when it is set to "Unrelease":
- Logged in to the group
- Participating in the folder
- Not subject to any viewing restrictions