1. A computer-implemented method of updating a virtual machine, the method comprising: executing a hypervisor on a host computer, the hypervisor managing execution of the virtual machine on the host computer, wherein a guest operating system is configured to execute on the virtual machine, the hypervisor configured to provide the virtual machine with block-based access to a virtual machine image, the virtual machine image comprised of blocks; receiving a substitution block at the host computer; and while the virtual machine is live, replacing, by the hypervisor, a target block in the virtual machine image with the substitution block, the target block comprising one of the blocks of the virtual machine image, the hypervisor managing the blocks of the virtual machine image and, when the virtual machine is executing, presenting the blocks of the virtual machine image, including the substitution block, to the virtual machine as a virtualized disk.