Microsoft Office Professional Plus 2013 | 1 x Combo Audio Jack |1 x VGA port | 1 x USB 3.0 | 2 x USB 2.0 | 1 x RJ45 | 1 x HDMI | 1 x SD/SDHC/SDXC/MMC Card Reader | Integrated 802.11 b/g/n WiFi | Bluetooth 4.0+ HS | 10/100 Base T | 30.3 x 20.0 x 2.17 cm | 3.08 lbs | For School and General Use
I think there are at least one (possibly more) specific 8.0 updates that need to be present before you can see the 8.1 upgrade via the store but I can't put my finger on it at the moment.
I know this or very similar issue has been addressed on this forum. You might want to go through this tutorial and see if any applies. I also suggest you try a search and look through some of the other tutorials.
Double check to make sure that you have all available Windows Updates installed, then check the Store for the update. The tutorial below can help show you more on updating from Windows 8 to Windows 8.1 Update.