First,
Check source.list follow http://pve.proxmox.com/wiki/Downloads
Second,
add non-subscription proxmox repo into source.list file (Package repository)
Last step,
#apt-get update #apt-get dist-upgrade
REF:
http://pve.proxmox.com/wiki/Roadmap#Proxmox_VE_4.2
http://pve.proxmox.com/wiki/Downloads
http://pve.proxmox.com/wiki/Package_repositories