To download and install the built-in help, first you need to know wich file to download. So, open LibreOffice, then choose menu
and see what is your LibreOffice version and operating system.
If you copy the info (like the selected text in the previous image) and paste it in a comment, it will be easier for us to guide you in installing the built-in Help.
EDIT: Now follow this link to download the file to install the Help for offline use matching your LibreOffice version. Once downloaded, with a double-click (or select and Enter) it will begine to install; accept or press OK when asked.
Then, when finished the installation and while in LibreOffice, if you press
F1 your internet browser will open to show the offline help (don’t require internet conection). Since the Help don’t cover all that could come to one’s mind, may be sometimes you will need to comeback to ask.libreoffice.
Edit your question if you want to add more information; also can comment an answer (Add Answer is is reserved for solutions).
Check the mark () to the left of the answer that solves your question. Pressing on your username near the top of this page, you can refer back to your previous questions and, if applicable, mark the correct answers.