If you're using a PC running Windows 7 or 8 [theregister.co.uk], you may be getting a little sick of endless popup screens [theregister.co.uk] telling you to upgrade to version 10. And you may be worried about inadvertently installing the upgrade as part of a security update.
Microsoft will start pushing out a Windows 10 upgrade as a recommended, virtually mandatory, update very soon (it's right now [theregister.co.uk] only an optional download). Some people are tempted to turn off Windows Update completely to avoid getting the new operating system – don't. It'll leave your computer vulnerable to attack as you'll no longer get security patches.
It's actually rather easy to turn off the Windows 10 upgrade function without losing vital regular software updates. Microsoft even has an official document [microsoft.com] explaining how to do it.
Make sure you follow all the steps, but essentially you have to:
1. Open the Registry Editor (search for regedit in the Start Menu and run it).
2. Set DisableOSUpgrade to 1 in HKLM\SOFTWARE\Policies\Microsoft\Windows\WindowsUpdate
3. Set ReservationsAllowed to 0 in HKLM\Software\Microsoft\Windows\CurrentVersion\WindowsUpdate\OSUpgrade
Or, the obligatory recommendation to run FOSS instead.