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On July 29, Windows 10 will progressively reach all users who opt for the update. Microsoft talks about million users waiting for the update.
For this they have reserved Windows 10 for free through the new icon that appears in the system tray. The truth is that really you don't even need to reserve Windows 10 to receive it for free.
Therefore, we must add those users who have not bothered to reserve the upgrade even though they can opt for itThere are also users who are in the Windows Insider program who will want, many of them, to update to the final version while others will stay with the beta.
Based on numbers, there may well be some 10 million users those who are interested in upgrading to Windows 10 from the outset. Note that Windows 7 and Windows 8.1 users can upgrade for free, and they are a good user base accounting for most of the market share. As for the number of users of the Windows Insider program, there are already no more and no less than 5 million participants.

Windows 10 will arrive gradually
These millions of users who have reserved the upgrade to Windows 10 will be the second to receive the upgrade. First up will be users of the Windows Insider program with Windows 10 Technical Preview.
Microsoft has reported that will arrive progressively to avoid problems with this Windows offered as a service. And so they can correct and solve problems on the fly. Remember that Windows 10 will be constantly updated.
In any case, those from Redmond hope that the problems will be minimal since there are many months of tests, feedback and they have verified that the majority of the users who have booked Windows 10 have compatible computers.