I am wondering the same thing. I need to delete a file in the iso so I can keep my apps from Windows 8.1 Preview. I DO NOT want to download over 100GB of data again.
. Sorry, you can only download the upgrade only from the windows store, however you can use a upgrade manger and choose keep apps, files, and user accounts. (Please note since you are using a free version of Windows you will have to buy Windows via the manger) so here is a CNET how to --->http://
and the Windows upgrade Manager (select save as iso when prompted)