Sign in to download from the App Store screen has the wrong Apple ID- grayed out. How can I change it?

Question

When I have a software update notification and I go to the 'Updates" tab of the App Store and click 'update',  the flash screen 'Sign in to download from the App Store comes up the the Apple ID is wrong and grayed out so I can't change it.  Where is the bogus Apple ID stored? How can I 'ungray' it to make it editable?
I'm running Mavericks 10.9.4 on an IMAC 2.5 GHz Intel Core i5.

Answer 1

You installed a hacked app, originally from the Mac App Store. It contains the receipt for a different app, downloaded using an account that you don't control. You need to identify and remove the hacked app.
Important: The app you need to remove is not necessarily the one named in the App Store alert. For example, the App Store may prompt you to update "Angry Birds" or "Twitter," but the hacked app may be something else entirely. Don't make any assumptions about which app you're looking for. To find it, you must carry out a systematic search with Spotlight.
1. Triple-click anywhere in the line of text below on this page to select it:
kMDItemAppStoreHasReceipt=1
Copy the selected text to the Clipboard by pressing the key combination command-C.
2. In the Finder, press command-F to open a search window, or select
          File ▹ Find
from the menu bar. In the search window, select
          Search: This Mac
from the row of tokens below the toolbar. Below that is a popup menu of search criteria, initially showing Kind. From that menu, select
          Other...
A sheet will drop down. In that sheet, select
          Raw Query
as the criterion, then click OK or press return.
Now there will be a text box to the right of the menu of search criteria. That's where you enter the raw search query. Click in that box and paste the text you copied earlier by pressing command-V.
4. The search window will now show all the App Store products that are installed. Compare those search results with the list of your purchases from the App Store. To see the complete list, you may need to unhide hidden purchases. If any apps were download from the App Store using other Apple ID accounts that you control, sign in to the store under each of those ID's and check the purchases.
At least one of the apps in the Spotlight search results is not among your purchases in the App Store. Move each such item to the Trash, after quitting it if it's running. You may be prompted for your administrator password. Empty the Trash.
Quit and relaunch the App Store. Test.
If you find these instructions confusing, ask for an alternative method.