Here's how you can upgrade to Windows 8.1 from earlier versions of Windows:
See if you are logged into your Microsoft account. If not, enter the Store, go to the Charms menu, select Settings, click the "Your account" option, and sign in. Then, select the large "Update to Windows 8.1 for free" tile in the Windows Store.
If this option isn't available, you may have to update your PC to the latest Windows patch. To do this, go back to the Start menu and enter the Charms bar. Next, select Settings, followed by the "Change PC settings" option, and click on Windows Update. Check for any new updates, install them, and restart your system.
If the Windows 8.1 download still isn't available, give it some time. Microsoft appears to be pushing the update out at different times.
Updating to Windows 8.1 from Windows 7 can be done through Microsoft's Upgrade Assistant. Windows 8.1 is available for $119.99, while Windows 8.1 Pro can be had for $199.99. The operating system can be purchased online as a digital download or in stores for a physical copy.
System requirements for Windows 8.1 are as follows: at least a 1GHz processor, 1GB of RAM for the 32-bit version or 2GB of RAM for 64-bit, 16GB of free hard-drive space for 32-bit or 20GB for 64-bit, and a graphics card capable of DirectX 9.
Windows settings, personal files, and most apps are saved when going from Windows 8 to 8.1, however only personal files are saved when upgrading from Windows 7.
Unfortunately, there is no easy path for those of you who have held out from updating to the latest Windows release. Windows XP and Vista users must do a clean install with a DVD copy of Windows 8.1. The operating system can be purchased at Microsoft Stores and retailers like Best Buy for $119.99 and $199.99 for the Home and Pro versions, respectively.
It is also highly recommended that you back up your files when upgrading. No Windows XP or Vista files or programs will be carried over to Windows 8.1.