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