KVM: Shrink Disk

Hello,

i have a running kvm virtual machine with proxmox 3. its running qcow2 and there is an 100 GB sized Windows 2012 installation inside.

i wanted to resize the disk and used "Resize Disk" inside Proxmox.

the size should be 120 GB but i made mistakes and now i habe 670 GB.

how can i set it to 120 GB without loosing data inside the vm?