There are actually pre-reqs to being able to install the Windows 8.1 update and also even being able to see it in your Store.
Firstly, you have to have a compatible operating system, below are the exceptions that will not be able to upgrade from the store:
- Windows 8 Enterprise
- Editions of Windows 8 Pro that are installed by enterprises using KMS activation
- Editions of Windows 8 that are installed using an MSDN ISO, and activated using multiple activation keys
If you installed Windows 8 using an MSDN ISO, you might be able to install Windows 8.1 using a similar ISO from MSDN.
Secondly, you need to have the following Windows Update installed on your machine:
If you have this update and you have a compatible version of Windows 8, then you should just click your store icon and update!