I don’t know how to download libreoffice help
The Help files downloads are on the same download pages where you got LO.
You need the Help file for the the same version of LibreOffice you have installed.
And it has to match - LO 32-bit needs 32-bit Help, LO 64-bit needs 64-bit Help.
You can find all LibreOffice downloads here:
https://downloadarchive.documentfoundation.org/libreoffice/old/
Find the folder for your installed LO version and look for the related Help file to download.
Install the Help file.