Microsoft has been more than generous and has provided completely free upgrade to Windows 10 to all the existing Windows 7 and Windows 8.1 users. This free upgrade offer was valid for one full year from July 2015 to July 2016. The offer has finally ended on the 29th July 2016. But you can still upgrade to Windows 10 for free if you use any of the Microsoft assistive technologies. What these assistive technologies do is that they guide you through the upgrade process and help you smoothly migrate from older version of Windows to the latest version of Windows for free.
Good news is that Microsoft has not announced any ending date for all the customers using assistive technologies to avail themselves a free upgrade to Windows 10. Microsoft mentions on the update site that they will announce such an end date when they will decide to end the free upgrade offer. It could mean that you still have many months for upgrading for free to Windows 10.
In order to get the free Windows 10 upgrade, you would need to use one of assistive tools like Windows 10 Upgrade Assistant. This method works only for Windows 7, Windows 8 and Windows 8.1. You can download this tool from http://go.microsoft.com/fwlink/?LinkId=822783 and launch it to start the upgrade process. The process involves scanning your system and determining whether your system fulfills the basic minimum requirements for running Windows 10.
If all the requirements are met, it begins to collect the data from your PC and then downloads Windows 10 files from the Microsoft servers. Some of the data from your older version of Windows will be kept while other will be discarded. After the download is finished, it will take another 20-30 minutes for the installation to complete and run Windows 10 on your PC.
You can find more information about the free upgrade to Windows 10 using assistive technologies from https://www.microsoft.com/en-us/accessibility/windows10upgrade.