Clicking the red cross on the right hand corner of the pop-up box now activates the upgrade instead of closing the box.
And this has caused confusion as typically clicking a red cross closes a pop-up notification.
The upgrade could still be cancelled, when the scheduled time for it to begin appeared, Microsoft said.
The change occurred because the update is now labelled "recommended" and many people have their PCs configured to accept recommended updates for security reasons.
This means dismissing the box does not dismiss the update.
Brad Chacos, senior editor at the PC World website, described it as a "nasty trick".
"Deploying these dirty tricks only frustrates long-time Windows users who have very valid reasons to stick with operating systems they already know and love," he wrote.
Microsoft said: "With the free Windows 10 upgrade offer ending on 29 July, we want to help people upgrade to the best version of Windows.
"As we shared in October, Windows 10 will be offered as a 'recommended' update for Windows 7 and 8.1 customers whose Windows Update settings are configured to accept 'recommended' updates.
"Customers can choose to accept or decline the Windows 10 upgrade."