Microsoft offered as a free upgrade to genuine Windows 7, Windows 8.1 and Windows Phone 8.1 customers.
How to prepare your PC for Windows 10 ?
- RAM: 1GB for 32-bit / 2GB for 64-bit
- Storage: 16GB for 32-bit / 20GB for 64-bit
- CPU: 1GHz or faster
- Graphics: DirectX 9 capable video card
Most important thing to check is windows updates.
Download Link Here.
Make sure first you backup your personal data on OneDrive or locally on your external hard drive.