After BITS completes the download, the install starts automatically usually on a restart or reboot. A few operations get underway then the EULA appears. If you reply 'yes' the install continues until W10 is installed. If you reply 'no' it auto reverts back to your previous OS. ... Not accepting the EULA doesn't disable the upgrade, though. As best I can tell (these are still early days), the EULA comes back on every reboot, even if you turned it down previously. Once the "Upgrade to Windows 10" update appears as checked on your PC, you're stuck in an endless installer cycle that kicks in every time you reboot. All of the installation files stay on your system, and Windows 10 is ready to be installed, again and again.
Juhůůůůůůůůů!