HiveMQ implements the logback framework and allows changes to the log configuration during run time.
It can be useful to increase the log level for HiveMQ or a specific extension. (See here how to configure a separate log level for extensions)
To change the log level simply edit the /opt/hivemq/confg/logback.xml file and adjust the level attribute in the associated <logger>.
Using the standard logback.xml file included in HiveMQ you can change the log level for the hivemq.log from INFO to DEBUG by changing the following line. (Line 69)
<root level="${HIVEMQ_LOG_LEVEL:-INFO}">
to
<root level="${DEBUG}">