http://m.kapilarya.com/fix-cant-download-windows-8-1-from-windows-store
Store
You need to use an account with Administrator privileges to install Windows 8.1, try logging in with the user account of an administrator of this PC and trying the upgrade again.
The error message says that you need to be logged in as administrator but when I got the error, I was already using the administrators account. The account I was using was in the administrators group and I can do other administrator functions with this account.
In order to fix this issue, you can follow these steps:
FIX : Can’t Download Windows 8.1 From Windows Store
1. Press
+ W and putupdates in Search box,
select View installed updates from search results.

2. In the Installed Updates window, look for the Update for Microsoft Windows (KB2871389) and right click on it, select Uninstall.

3. Now using following links, download the KB2871389 update package manually for your Windows 8 edition and install it:
Download Link [All supported x86-based versions of Windows 8]
Download Link [All supported x64-based versions of Windows 8]
Download Link [All supported x64-based versions of Windows Server 2012]
After following above mentioned steps, you can re-try to download Windows 8.1 from Windows Store, it should work without any issues.
Hope this helps!