If you have had it for more than 30 days, you will have to get your product key (use a program like produkey - it sometimes gets flagged by antivirus software but it is safe) and then install Windows 8.1 using installation media, which you can get here: http://windows.microsoft.com/en-GB/windows-8/create-reset-refresh-media
If you have had it for more than 30 days, you will have to get your product key (use a program like produkey - it sometimes gets flagged by antivirus software but it is safe) and then install Windows 8.1 using installation media, which you can get here: http://windows.microsoft.com/en-GB/windows-8/create-reset-refresh-media