It’s NOT on the Help menu! and F1 only works for Win support, not any program-specific support. (I have a HP Envy Notebook). Gosh, it’d be so easy if the help index was available on the Help menu!
You need to get the packaged offline help for your OS and for your user-interface-language as you got the LibreOffice installable package. Then to install it.
The files will be placed in your LibreOfficePath/help. At least this is the location for Win OSes.