If you’re on Windows 7 or Windows 8.1
Warning: this guide is designed for the technically minded, so if you just want to upgrade to Windows 10 then please follow the usual process.
First things first, backup everything you want to keep on your PC. You’ll need to upgrade to Windows 10 first before a clean install. This is the safest way to ensure Windows 10 recognizes your product key and upgrades you correctly. You can chance clean installing without the upgrade path, but Microsoft does warn it might not be able to detect the product key from your PC’s motherboard. That means upgrading the supported way, and not with an ISO. There are two ways to do this officially and retain your license and activation properly. The first is using the usual reservation method detailed here, but skip that and use Microsoft’s method to bypass the line. Visit Microsoft’s Windows 10 download page and select the 32-bit or 64-bit installer that’s relevant to your current operating system. Once the tool opens you’ll have two options: upgrade this PC now or create installation media for another PC. Select upgrade this PC and follow the prompts.
Windows 10 upgrade prompt
Once you’re on Windows 10, ensure that you login with your Microsoft Account you intend to use. Check that your PC is activated and check the copy of Windows 10 you have installed from Settings > Update & Security> Activation. Write down the exact copy of Windows 10 you have installed, and then download the Windows 10 installer again on your machine and this time select the second option: create installation media for another PC. You’ll need a USB key that’s at least 3GB in size or the ability to burn an ISO file to a DVD later. When you pick the version of Windows 10 ensure that it matches the version you noted down after the upgrade. Once your USB key or DVD is ready, reboot and use the BIOS options on your PC to boot from the USB or DVD. There will be a variety of setup prompts, but ensure you choose the custom installation to format your existing partition. Ignore any product key prompts along the way by selecting the skip option. Once you’ve installed Windows 10 it should automatically activate, but if it doesn’t then you can run slmgr.vbs /ato from a command prompt to initiate activation.
If you’re already on a Windows 10 preview machine
If you’ve been testing Windows 10 and now you want to clean install it on your machine, things get a little tricky. First things first, backup everything you want to keep on your PC. To be 100 percent sure you’ll retain your product key you’ll need to restore your PC back to Windows 8.1 or Windows 7 and then perform the upgrade process detailed above. You can restore your PC using the DVD your PC maker supplied with your PC, or attempting the reset PC process on your machine. This may have been overwritten by Windows 10, so if you’re not able to restore then contact your PC maker. You can try going for a clean install using the ISO, but it’s not guaranteed to work unless you’ve officially upgraded that machine to the final version of Windows 10 from a Windows 7 or Windows 8.1 install. I tried clean installing and I lost activation, but restoring to a Windows 8.1 install to upgrade to Windows 10 and then format and clean install worked. It’s a little messy for Windows 10 testers, but it’s the guaranteed way to ensure you don’t lose activation.