An icon should be present on your Win7/8 task bar which lets you reserve a copy. It does seem to be a gradual rollout though, as mine still tells me it's not available yet.

It's probably quicker to download directly via The Windows 10 Download Tool: http://www.microsoft.com/en-us/software-download/windows10

I can confirm that. I didn't see anything about Windows 10 in the Windows Update window, but upgrading via the tool worked perfectly for me.

Do you have to burn it to a USB/CD as the page suggests, or is does the tool give a way to upgrade in place without doing that?

Edit: can confirm crazysaem's experience. I did an in-place upgrade on a fully-patched 8.1 system and it worked flawlessly and relatively quickly (I kept my apps and settings). I'm not a heavy Windows user though.

You have the option to either create the USB/CD media or do an in-place upgrade. I did the in-place upgrade, which worked without problems for me. They also provide the option to either keep your apps and files or delete them. I chose to delete them, because I wanted a clean install.

Assuming you've clicked the Reserve-button in the little notification application that was rolled out via Windows Update, you'll just have to sit tight and wait.

Check out the FAQ: http://www.microsoft.com/en-us/windows/windows-10-faq

Why reserve something for which scarcity is impossible?

Their capacity to upload copies of Windows 10 is not infinite (though only because they have chosen not to distribute via BitTorrent).

It's a soft release. There is actual scarcity depending on what device you have.