  The GERDA collaboration is performing a sensitive search for neutrinoless
double beta decay of $^{76}$Ge at the INFN Laboratori Nazionali del Gran Sasso,
Italy. The upgrade of the GERDA experiment from Phase I to Phase II has been
concluded in December 2015. The first Phase II data release shows that the goal
to suppress the background by one order of magnitude compared to Phase I has
been achieved. GERDA is thus the first experiment that will remain
background-free up to its design exposure (100 kg yr). It will reach thereby a
half-life sensitivity of more than 10$^{26}$ yr within 3 years of data
collection. This paper describes in detail the modifications and improvements
of the experimental setup for Phase II and discusses the performance of
individual detector components.
