On Windows 8, you can do this by pressing Windows key+C > Settings > Change PC Settings.
Now click Update and recovery on the left pane. Hit the download or install button to update your PC.
On Windows 7, you can do this by clicking on the Start button, type update in the search box and click Windows Update.
In the left pane, click Check for updates. Now select all updates and click Install updates.
After the updates are installed, the Get Windows 10 app should be installed on your computer. This app lets you reserve your copy of Windows 10. Here’s how.
Click the Windows icon on the system tray.
Click Reserve your free upgrade.
Enter your email address to receive confirmation of your reservation.
Now you’ll see a message that says your upgrade has been reserved. There’s aCancel reservation button on the bottom left here. You can click that if you wish to cancel your reservation.