The Pierre Auger Observatory has collected the largest exposure of ultra-high energy cosmic rays over the past decade, providing an unprecedented data set of high quality. The analysis of these data has led to major breakthroughs in the understanding of the origin and properties of the highest-energy cosmic rays, but a coherent interpretation has not yet been achieved. New questions have emerged, including that of the composition of cosmic rays in the energy region of the flux suppression, which is of key importance for making progress. To answer these open questions, the Observatory has started a major upgrade, called AugerPrime. The upgrade program will include new plastic scintillator detectors on top of the water-Cherenkov detectors of the surface array (SD), a more powerful SD electronics and the extension of the dynamic range with an additional PMT installed in the water-Cherenkov detectors. The main goal of AugerPrime is to improve the mass composition sensitivity using the surface detectors. At the end of 2016 an Engineering Array of the upgraded detectors has been installed and has taken data since then. After reviewing the physics motivation of AugerPrime, an overview of the different parts of the upgrade will be given. The expected performance and the improved physics sensitivity of the upgraded Observatory will be discussed together with the first data collected with the Engineering Array.