With the official announcement of Windows 10 is to be released on the 29th of July, Microsoft is planning to do one of the biggest deployment in history. For the first year Windows 10 will be offered free and as per the information available today, it will be done through a Windows update (something similar to the update done from Windows 8 to Windows 8.1).
For devices running Windows 7 SP1 and Windows 8.1, Microsoft will offer this free upgrade. In order to get the free Windows 10 upgrade, first you will need to reserve your free upgrade. Through Windows update your devices should have the Get Windows 10 app installed.
The Get Windows 10 app visible in the system tray at the bottom right-hand side of your screen
Click and open the Get Windows 10 App. Read the important information which I have highlighted and click on reserve your free upgrade
Type in your email id to get the confirmation and click on send confirmation
Your Windows 10 is reserved. Click close and wait for July 20th