Microsoft Office Home & Student 2013 is $139.99, which includes Word, Excel, Powerpoint, and OneNote. An alternative is Office 365 Personal, which is $69.99 for a one year subscription.
If you were asking about upgrading the operating system to Windows 8.1, the cost of an OEM license for Windows 8.1 Professional is currently on sale for $96.99. Just run the Windows upgrade tool to make sure your hardware is compatible before purchasing, and backup your data before installing. http://windows.microsoft.com/en-US/windows-8/upgrade-to-windows-8#
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Microsoft - Windows 8.1 64-Bit - System Builder (OEM) - DVD-ROM - English
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