You have already reserved your Windows 10 upgrade and received the prompt on your system that the upgrade is ready to begin.
The prompt offers you the opportunity to schedule the upgrade over the following 72 hours or to start it immediately.
If you pick the later then the upgrade will begin immediately and there is no getting away from it. On the plus side you can always use the Revert option in Windows 10 to revert to your previous operating system if you decide Windows 10 is not for you.
If you schedule the update sometime in the next three days, then you can still stop it from happening by following these steps:
1. Uninstall KB3035583 and select the option to reboot later.
2. Run the Disk Cleanup tool and remove the installation files.
3. Wait for the scheduled time to arrive because that has alreay been set in motion if you choose a time for the upgrade. One hour prior to that you will see a window on your screen indicating that it is almost time for the upgrade to happen. This dialog does offer you the chance to reschedule as well.
4. Once the timer reaches zero then the system will begin the upgrade process.
5. Following the reboot, the attempt to upgrade will fail because you removed the temporary installation files earlier in this scenario.
6. You now need to check Windows Update and hide KB3035583 and the Windows 10 Upgrade when it is offered so they will not start again.