If you are on Windows 8,
1. Press Windows key+C > Settings > Change PC Settings.
2. Click on Update and Recovery.
3. Click on Download or Install button.
These steps will install latest Windows updates on your PC.
Now install Get Windows 10 app on your PC.
4. Click on Windows icon on system tray, and click on Reserve your free upgrade. Enter your email ID to receive confirmation of the reservation.
If you are on Windows 7,
1. Click on Start button.
2. Type Update in the search box and click on Windows Update
3. Click on Check for updates, select all of the updates and click on install updates.
This will install latest Windows updates on your device. Rest of the instructions are similar to the above for Windows 8.
Live....and Let Live!