There are several ways how to set the default JRE, but this is only documented in source code.
Please add this information to the help page.
The standard JRE can be set:
* By setting the environment variable JAVA_HOME
* By adding a JRE to the $PATH variable
* By providing a configuration file like in the user profile (config/javasettings_*.xml in <instdir>/presets/config (e.g. on Ubuntu /usr/lib/libreoffice/presets/config)
This is the page: https://help.libreoffice.org/Common/Advanced