A flood of reports from across the web say the Anniversary Update is rolling out to a wide swath of Windows 10 installations today.
Many people who check right now will see that Win10's Windows Update (Start > Settings > Update & security > Windows Update, click Check for updates) lists "Feature update to Windows 10, version 1607." If you install that update/upgrade/patch, your Win10 Fall Update system (version 1511) will be upgraded to the Anniversary Update (version 1607).
If you see the "Feature update" lurking and want to take your time installing it, there's a very simple series of steps you can take right now to block it. But unless you've blocked Win10 upgrades (with the Win10 Pro Defer upgrades setting), you need to take those steps now, before the update gets installed.
Here's how to hold back on the Anniversary Update. It'll work regardless of which edition of Windows 10 you're using, as long as you aren't connected to a corporate update server (in which case your admin should be on top of all this).
Step 1. Go to KB 3073930 and download Microsoft's Wushowhide tool. (Click the link marked "Download the 'Show or hide updates' troubleshooter package now.") Drag the downloaded file, Wushowhide.diagcab, to any convenient location.
Step 2. Double-click on Wushowhide.diagcab to run it. Click the link marked Advanced. Uncheck the box marked "Apply repairs automatically." Click Next.
Step 3. Wushowhide will run for a long, long time. When it comes back up for air, click the link to Hide Updates. If you see a checkbox marked "Feature update to Windows 10, version 1607," check the box next to the item and click Next. (If you don't see "Feature update to Windows 10, version 1607," the upgrade isn't being sent to your box yet. Check again tomorrow.)
Wushowhide is an odd bird. If it successfully hid the upgrade/update/patch, you see the "Troubleshooting has completed" dialog shown in the screenshot.
Step 4. Click Close. You're done.
If you don't trust Microsoft's wushowhide tool, you can verify for yourself that it hid the Anniversary Update. Go back to Windows Update (Start > Settings > Update & security, then Check for Updates) your machine should show "Your device is up to date." The Anniversary Update didn't get installed, as you can verify by typing winver down in the Cortana search box.
When you're ready to install the Anniversary Update -- you probably will, at some point -- the reverse procedure's just as easy.
Step 1. Double-click on Wushowhide.diagcab to run it. Click the link marked Advanced. Uncheck the box marked "Apply repairs automatically." Click Next.
Step 2. Wushowhide will run for a long time. When it comes back up for air, click the link to Show hidden updates.
Step 3. Check the box next to "Feature update to Windows 10, version 1607," click Next, and click Next again.
Wushowhide will dutifully tell you it is "Resolving problems." When it's done, you see the same "Troubleshooting has completed" dialog as shown in the earlier screenshot.
Step 4. Click Close.
The Anniversary Update will get queued up again and the next time Windows Update runs (you can Check for updates manually, or let it run by itself, likely overnight), your machine will reboot into Windows 10 Redstone 1, version 1607, Anniversary Update, build 14393 - Win 10.2, to my way of thinking.
By the way, in case you missed it, if you were upgraded to the Anniversary Update and you want to go back to the old Fall Update, you have 10 days from the AU installation date to perform an automatic rollback. (If you were automatically upgraded any time after the release on August 2, you still have time.) Click Start > Settings > Update & security. On the left choose Recovery. On the right, under "Go back to an earlier build" click the box marked Get Started.