Since it hasn't been pointed out, it's also possible windows will download the Windows 10 upgrade without having to reserve it. This wasn't the original article that I have read about it, but this also includes a link to how to remove the download.
So reserved or not, you may still have the 6GB download on your computer.
Edit: The first article says this about the folder.
According to the Inquirer, a user who had never “reserved” a copy of Windows 10 in the first place found a large 6GB download sitting in the $Windows.~BT hidden directory, and a series of failed “Upgrade to Windows 10″ tasks in Windows Update’s history. In several cases, the new OS has been downloaded over metered connections, forcing people over their bandwidth caps in the process. When the Inquirer reached out to Microsoft, the company said the following: “For individuals who have chosen to receive automatic updates through Windows Update, we help upgradable devices get ready for Windows 10 by downloading the files they’ll need if they decide to upgrade.
The section on deleting the folder shows this:
In Windows 7, you do this by clicking on “Tools,” then “Folder Options,” and finally “Show Hidden Files and Folders,” as shown below. In Windows 8/8.1, click on the View tab and then select the “Hidden items” check box.
Once this is done, check your Windows directory for a directory named $WINDOWS.~BT. The icon may be translucent, since the folder is normally hidden, so check carefully. You can delete this folder if you wish, but doing so won’t actually prevent Microsoft from downloading the setup program again. Once the OS has decided that you’re going to install Windows 10, it’s downright pushy about having the data locally. The only solution, according to various sources, is to actually remove a specific Windows Update: KB3035583.
KB3035583 is described by Microsoft as installing “the Get Windows 10 app, which helps users understand their Windows 10 upgrade options and device readiness.” It can be uninstalled by navigating to Windows Update from within the Control Panel, choosing “Programs and Features,” and then selecting the “View Installed Updates” option. Remove this update and then delete the folder, and you’ll reclaim your lost disk space.
KB 3035583 can then be blocked from installing again by hiding the update from within the Windows Update setting in Control Panel.
There are facets to this situation that aren’t fully understood as yet. My own Windows Update history shows that I installed KB3035583 on the 26th of July, as shown below.
Despite this, there’s no sign that my system ever downloaded Windows 10, and I have no record of failed W10 installations (another reported commonality) in my own Windows Update history. In some cases, this MS update clearly triggers a download process, but in others, it does not seem to do so. I personally run Windows 7 Professional, but IE11 and Windows Update have both been incessantly nagging me to upgrade.
One potential reason for this is that I keep Windows set to “Check for updates but let me choose whether to download and install them.” It’s possible that this setting keeps Windows 10 from downloading whether you’ve installed KB 3035583 or not.