My home computer (just purchased) running Windows 8 tells me I need to use the account of an administrator to update to Windows 8.1. I am the only one who has a user account. Who is the administrator?

To check if you are logged into Windows 8 as a user with administrative rights, please:

a) Right-click in the bottom left corner of your screen, and then click Control panel.
b) open User accounts
c) click "Manage another account"

There will be all user accounts listed on the page. Each account has assigned some status (Administrator, Standard user, etc.). Before installing or reinstalling AVG, or prior to proceeding with any action that requires administrative access, please make sure that you are logged into Windows under an account with the status "Administrator".

If you are not sure under which account you work at the moment, please open the main User Accounts page. The current user is displayed in the right part of the page. Follow these methods:

Method 1: You may try creating a new Administrator User account and check if this helps to upgrade to Windows 8.1.

User accounts: frequently asked questions: http://windows.microsoft.com/en-us/windows/user-accounts-faq#1TC=windows-8

Method 2:
You may also follow the suggestion provided by "Anon" from this link which is similar to the issue you are facing and which has been marked has Answer.

http://answers.microsoft.com/en-us/windows/forum/windows8_1-windows_install/windows-81-will-not-download-from-the-store/d3c57b8b-acd2-4bd9-82e4-1b51b206e311