  The GERmanium Detector Array GERDA experiment, located at the Laboratori
Nazionali del Gran Sasso (LNGS) of INFN, searches for $0\nu\beta\beta$ of
$^{76}$Ge. Germanium diodes enriched to $\sim 86~\%$ in the double beta emitter
$^{76}$Ge ($^{enr}$Ge) are exposed being both source and detectors of
$0\nu\beta\beta$ decay. Neutrinoless double beta decay is considered a powerful
probe to address still open issues in the neutrino sector of the (beyond)
Standard Model of particle Physics. Since 2013, just after the completion of
the first part of its experimental program (Phase I), the GERDA setup has been
upgraded to perform its next step in the $0\nu\beta\beta$ searches (Phase II).
Phase II aims to reach a sensitivity to the $0\nu\beta\beta$ decay half-life
larger than $10^{26}~$yr in about 3 years of physics data taking. This exposing
a detector mass of about $35~$kg of $^{enr}$Ge and with a background index of
about $10^{-3}~$cts/(keV$\cdot$kg$\cdot$yr). One of the main new
implementations is the liquid argon scintillation light read-out, to veto those
events that only partially deposit their energy both in Ge and in the
surrounding LAr. In this paper the GERDA Phase II expected goals, the upgrade
work and few selected features from the 2015 commissioning and 2016 calibration
runs will be presented. The main Phase I achievements will be also reviewed.
