to upgrade windows 8 to 8.1 you will need to first download all updates for windows 8 - This could be up to 3gb worth of updates which can be downloaded from the windows update and recovery section in pc settings
once all updates have been installed and the computer restarted to finish installation process you can then update windows 8.1 from the windows store
if it does not show either you havent installed the required updates or theres some other problem
Please note the following windows 8 do NOT support upgrade
Windows 8 Pro, if installed by an organisation or a program that uses Volume Licensing. Talk to your system admin, or the administrator of the program that was used to install Windows 8. If you’re the administrator, you can find more info and links to Windows 8.1 ISOs in the Windows 8.1 Enterprise Evaluation.
Windows 8 Pro, if installed using an MSDN or TechNet ISO, and activated using Multiple Activation Keys. You might be able to download a Windows 8.1 ISO from MSDN or TechNet.
see installation instructions
see why cant I find the update in the store