I have to update some DELL Optiplex 3010 PCs with a pre installed Windows 8 prof System
after completing the pre Installation where I only had to choose the language type, a PC Name and a user Name and Password, i invoked the store ICON comming up with a big window whre i can choose to update to Windows 8.1Pro
After a click on the button Download I get the Messge
You Need Administrator rights for installing Windows 8.1 ( I get this message in german language, so the translation might be different)
the user Name I used is a member of the local Administrator Group
I also tried to activate the Administrator account Logging in as Administrator I get a message
like It is not allowed to use the Administrator account for the Update
Otherwise it should work - I did just as you did. Turn the computer all the way off, restart and try again.