The LibreOffice online help (wikihelp) is generated from the same source that is used to build the help you can install on your computer. So when you want to improve / fix the wikihelp, you actually have to improve / fix the application help itself.
It is in this repository: http://cgit.freedesktop.org/libreoffice/help/
You can contribute the same way as you’d do with code contributions - please see how to setup the access for you here: https://wiki.documentfoundation.org/Development/gerrit
Unfortunately the above is not as straight-forward as editing the help in a real ‘wiki’ way would be; we haven’t yet managed to switch the source the other way around - that the wiki would be the source, and the offline help would be generated from that.
If you get stuck, please go to the IRC (irc.freenode.net), channel #libreoffice-dev, and the people there will help you for sure. Thank you for your interest in helping! - hope the above will work for you