I copied and pasted the steps set forth below from another thread. The procedure worked for me on both of my 8700s after I had tried just about every other proposed solution. Hopefully, the procedure will work for you as well (assuming you haven't already tried it).  1) Uninstall KB2871389 and reboot.  2) Download the KB2871389 update from Microsoft --> [URL] 3) Install the KB2871389 update manually and reboot. 4) You should now (hopefully) see the 8.1 upgrade in the Windows Store.
My Dell XPS 8700 desktop arrived yesterday but I cannot get it to upgrade from Windows 8 to Windows 8.1. I've been to several Microsoft websites where they tell me about a Windows update and I have to have before the upgrade will appear in the Microsoft store. I have verified that I have done all available Windows updates includingthe necessary for the upgrade to 8.1. I tried running "wsreset.exe" to clear the Windows store cache. This is not an enterprise system. It's an ordinary Dell OEM version of Windows 8 that came with the machine. No matter what I do the update does not appear in Windows store. I thought perhaps since I was using the same Microsoft account for my laptop which I already upgraded from 8 to 8.1 that perhaps it thought I was already upgraded. So I created a new Microsoft account, created a new user, remove the old user, and tried to get the upgrade under a fresh Microsoft account name and user. Still no luck. I've seen one other person post a message on a forum somewhere who had a similar problem on this exact same model that the only reply that got was to go to the Microsoft page that tells you to do all of the things I have already done. Any suggestions or websites where I could crosspost this problem would be much appreciated.
Windows 8.1 does not show in Windows store Dell XPS 8700