To restore a system image backup, you’ll need to boot from an external source such as a "Recovery USB"

Changing the boot order is different on every Desktop/Laptop/Tablet. Consult the manufacturer.
For example my tablet requires holding down the up volume immediately after hitting the power on.

Insert your Recovery USB and reboot your computer.
When the installation process Starts,
   click the Repair your computer link.

Click the Troubleshoot tile

Click the Advanced Options tile

  If you feel your Recovery USB backup is a bit old and only wish to back the bus up a little you may try to
  select the tile System Restore to use a restore point recorded on your PC to restore windows

Select the System Image Recovery option to re-image your computer
from the system image on your Recovery USB.