Not doing great, I've been trying to get this posted for the last week on the off-hand chance it isn't old news & it helps anyone out...
To upgrade win 8 to 8.1 you're supposed to download/install the update through the store. Google & errors, including not very helpful something happened sort of explanations, are a bit common, as are problems not having the upgrade displayed in the store itself. Note that just because your system runs win 8 does Not mean it'll run 8.1 -- hardware requirements are stiffer. Upgrading through the store can work however, & I'm unsure if there are any benefits or if it's just a case of Microsoft being jerks -- I'm guessing in theory that the interaction between MS servers & the setup process on your hardware ***Might*** make for a better upgrade than a more closed off process running off an ISO etc., but whether that's true or not I've no idea. At any rate, downloaded files are stored in a hidden folder at root [C:\] level, & if you don't restart the 1st time the upgrade prompts you, you can copy that folder elsewhere [obviously restart after the copy]. Inside that folder are the setup files, But, they won't install or anything on their own since the win 8 &/or placeholder keys do not work -- more on the key stuff later. Most of the files in that hidden folder go away after install, and you are not left with any setup files, meaning win 8's reset features are not available etc.
OK, getting an ISO... Google win 8.1 ISO & you'll get hits like this one -- http://winsupersite.com/windows-8/windows-81-tip-download-windows-81-iso-windows-8-product-key
In a nutshell running win 7 you download the Microsoft installer for 8 & 8.1, enter your win 8 key in the win 8 downloader/installer & start the download process, kill the app, starting the 8.1 downloader/installer. The 8.1 version will not take your win 8 key, so it uses the win 8 installer to authenticate whatever. How long this will continue to work is anyone's guess -- Microsoft could tighten things up or just release ISOs making it all unnecessary. When the download completes you've got 2 folders -- C:\ESD & C:\Users\[UserName]\AppData\Local\Microsoft\WebSetup -- where the setup files & the app to install, create ISO, USB stick etc. are stored. If those folders are still there from a win 8 download, they have to be deleted before you run the 8.1 downloader, or else all you'll get are the same win 8 files. From win7 After you complete the win 8.1 download you can create the ISO, run the install, create a USB stick etc. whenever you want by running the Install Windows app, with it's default desktop icon. The downloader/installers would not work for me in a VM, & the 8.1 version won't run in XP -- v. 8 will. I did not try running the win 8 installer in win 8.
Once you've got the setup files downloaded... There are 2 placeholder keys available -- 334NH-RXG76-64THK-C7CKG-D3VPT for Core & XHQ8N-C3MCJ-RQXB6-WCHYG-C9WKB for Pro. The pro key worked for upgrades running win 8, & for fresh installs -- add/replace the key with your win 8 key once it's running. Alternatively Google and you'll find info on adding a text file named ei.cfg to the sources folder in the ISO [using Daemon Tools or similar] or on a setup USB stick you created -- this lets you skip adding a key during fresh install, but will not let you upgrade win 8 -- it ***May*** be the way to get a trial version, but I'm not 100% sure yet. The text file contents I tested are:
[EditionID]
[Channel]
Retail
[VL]
0
Microsoft updated the Windows Disk Cleanup tool -- under Accessories in win7's Start Menu & in Admin Tools in win8/8.1. Cleaning up, getting rid of old setup & update files is generally worth it, whether you're in 7 or just installed/upgraded to 8.1 or whatever.