Upgrade from Windows 7, 8, or 8.1 to Windows 10 entirely for free and using the official Microsoft web site.

Web site where you can upgrade to Windows 10:
https://www.microsoft.com/en-us/softw...

Microsoft is no longer supporting Windows 7. If you're still running Windows 7, you're at risk of attacks and viruses since there are no longer any updates or patches. Windows 8 and 8.1 are both still supported; however, since the upgrade is free, you might as well get onto the latest and greatest version of Windows.

On the web site under the Create Windows 10 installation media, click on Download tool now. This will download an exe to your computer. Click on this file. When finished, open the download and accept the license terms
Choose: 'Upgrade this PC now' then click ‘Next’
Choose ‘Keep personal files and apps’
Once installed, open: Settings - Windows Update - Activation to activate your Windows 10 digital license...
OR enter your (genuine) Windows 7 or Windows 8/8.1 product key if you haven’t previously activated your old version of Windows

If you’d like to check your license has successfully been activated afterwards, open: Settings - Update & Security - Activation.

Note: a Windows 10 license is tied to your hardware. As such, if you need to format your computer for any reason Windows 10 will reactivate automatically.

As full disclosure, I work at Microsoft as a full-time employee.