by Visvanath Ratnaweera.
Recommend the Upgrading tutorial to your IT.
You didn't mention the source release. Depending on it worth checking the limitations as summarized in this chart http://www.syndrega.ch/blog/#php-and-dbms-compatibility-of-major-moodle-releases.
If the chart is too curt, recommend this discussion Upgrading from Moodle 4.0.4: Need Advice on Intermediate Steps too.