Just for the record, you probably would be better off biting the bullet and upgrading to Windows 10, but if you don't want than there are a couple of options.
1) As Ramoss said, install the GWX Control Panel. Infoworld has a
page about the steps to take to do so.
2) Microsoft has a
page describing how to do so but it involves editing the registry and that can be a bit scary as you can render your computer inoperative if you mess it up.
I guess there are really is no good path for you to take on this as they all involve messing around with your computer settings.
Edited by kjsimons, : No reason given.