Specifically:
https://www.microsoft.com/en-us/windows/windows-10-faq
Can I get my free Windows 10 upgrade even if I didn’t reserve?
Yes. The easiest way to get the free upgrade is to reserve, but you can upgrade even if you don’t reserve.
Once Windows 10 is available, we’ll begin notifying devices that their free upgrade is available. Just open the Get Windows 10 app to schedule your upgrade. Note: Some notifications will go out as soon as Windows 10 is available; others may go out in the weeks or months following.
Why don’t I have the Get Windows 10 app?
If you don’t see the Get Windows 10 app on your PC, it might be because:
Your device isn’t up-to-date with at least Windows 7 SP1 or Windows 8.1 Update.
Windows Update is turned off or is not set to received updates automatically.
You’ve blocked or uninstalled the necessary Windows Update functionality.
Your device is not running genuine Windows.
Running Windows Update and installing any available updates will fix the first three issues. Need more help? Go to our Windows Community Forum.
PCs that we determine cannot run Windows 10 will not see the Get Windows 10 app before July 29, 2015. After July 29, 2015, we’ll enable the icon in the system tray. This is to help ensure that you can easily check your PC’s compatibility if you choose.
If your device is managed as part of a school or business network, please check with your IT administrator about upgrading to Windows 10.