If you have Windows 8 and the update option is not shown in the Store that is an indication that Windows 8 need some maintenance done.
Look for a notification (right end of taskbar). Most likely there are some items indicated that need your attention. Run a Defender scan (quick scan will do). Make sure it says “PC status: Protected”.
Go to Control Panel > System and Security > Action Center. Check in the Maintenance section for any items that need attention and take care of them.
Check for any updates needed. Go to Control Panel > System and Security > Windows Update.
If you see any updates available click on the message line and install the updates. When your Windows 8 system is all up to date (restart if needed), go back to the Store. You will then see the invitation to update to Windows 8.1.
Note the process takes some time. For the Windows 8 standard version the download is 2.81 GB. If you have Windows 8 Pro installed the download is 3.63 BG. So depending on your Internet speed this can be a lengthy task.
For more on the update process see How do I upgrade to Windows 8.1 ?
© 2013 Ludwig Keck