Microsoft recently made the Windows 10 upgrade a recommended one meaning if your settings in Windows updates is set to automatically install then you will automatically upgrade to Windows 10.
Here's a few tips from Winbeta.com on how to avoid an unwanted upgrade:
Reference
automatic install
control panel
pc settings
recommendedupdate
security updates
settings
software management
system maintenance
tech support
update control
update notifications
update tips
upgrade
upgrade process
user guide
windows 10
windows 7
windows 8.1
windows update
