The upgrade of the Pierre Auger Observatory ---AugerPrime.

Over the last 15 years the Pierre Auger Observatory has accumulated the world's largest exposure to ultrahigh-energy cosmic rays (UHECRs) and the analysis of this dataset led to major advances in our understanding about the nature of UHECRs. The new perspectives opened by the current results call for an upgrade of the Observatory (dubbed AugerPrime), whose main aim is the collection of new information about the primary mass of UHECRs, mandatory to interpret all the observations in a unified picture. The upgrade program includes: the installation of a plastic scintillator detector on top of each water-Cherenkov detector (WCD) of the surface array; an extension of the dynamic range of measurement through an additional small photomultiplier tube in each WCD; an array of underground scintillator detectors to measure the muonic component of extensive air showers; the deployment of a radio antenna atop each WCD; new electronics to process the signals from all the detectors. An overview of the upgrade is provided, together with the expected performances and the improved physics sensitivity.