Find the file [netbeans installation directory]/etc/netbeans.conf
Luckily, Linux has a find helper like
find /home/ -name "netbeans.conf,
in which you can change the /home/ to a location where you want to search.
I found it at /usr/local/netbeans-8.1/etc/netbeans.conf
Once, you found the file, the following property needs to be set:
netbeans_jdkhome="[jdk_path]"
where you can find the jdk_path using:
update-alternatives --config java
In my case, I found it at /opt/java/jdk1.8.0_191