Add the Proxmox VE repository key:
wget http://download.proxmox.com/debian/proxmox-ve-release-5.x.gpg -O /etc/apt/trusted.gpg.d/proxmox-ve-release-5.x.gpg chmod +r /etc/apt/trusted.gpg.d/proxmox-ve-release-5.x.gpg # optional, if you have a changed default umask
Update your repository and system by running:
apt update && apt dist-upgrade
