A: Each Microsoft ID that's used to purchase applications from the Windows Store can be associated with up to five machines. If you used other machines with your Microsoft ID and no longer use them, you can disassociate them to free up one of your five slots. To disassociate a machine, perform the following.
  1. Open the Windows Store application.
  2. Open the Settings (Windows Key+C, then select Settings or just swipe from the right side of the screen) and select Your account.
  3. All your PCs will be displayed. Those no longer needed can be removed by clicking the Remove button under the machine name. (See screen shot below.)
winstoreaccountsml
Windows Store Accounts Setting

You might also want to know how to force a Windows 8 or Windows RT update. The FAQ "How can I force an update of my Windows 8 or Windows RT environment?" can show you how.