Upgrading from 5.6.8 to 6.8

Hi, I'm having some trouble upgrading elasticsearch.
Following : https://www.elastic.co/guide/en/elasticsearch/reference/6.8/rolling-upgrades.html
And: https://www.elastic.co/guide/en/elasticsearch/reference/6.8/deb.html

But I get :
warning: Falling back to java on path. This behavior is deprecated. Specify JAVA_HOME
ERROR: keystore does not exist at [/etc/default/elasticsearch.keystore]
dpkg: error processing package elasticsearch (--configure):
subprocess installed post-installation script returned error exit status 78
Errors were encountered while processing:
elasticsearch
E: Sub-process /usr/bin/dpkg returned an error code (1)

I've tried setting JAVA_HOME in: etc/default/elasticsearch as:
JAVA_HOME=/usr/lib/jvm/java-1.8.0-openjdk-amd64

But not quite sure how to get it right... Anyone got a clue?

Thanks!