screenshot from microsoft.com
To get free upgrade you can go to this link https://www.microsoft.com/en-us/windows.
But before you upgrade, you need to check your PC/desktop/notebook that meet the requirement for Windows 10.
I've use this windows 10 about 1 year, since 2015 when the Windows 10 free upgrade offer starting.
OK, if you already have Windows 10, you will be get the 1st year anniversary upgrade.