Download the ISO of the current build and ugprade it
Search on the internet for "10240 ISO" and download it. These are actually leaked ISOs, but you should be able to use them without any problems. Build 10240 is a RTM build, which means that it's activated if you've got a valid Windows 7/8.1 key and it does no longer send all your keyboard inputs to MS.
I don't know if I can post a link, because it had been published on a site, which is hosting illegal/cracked software, too. Actually this site is a very good source for unmodified Windows ISOs.
Just for your information: I don't use any cracked programs and I wouldn't like to do so.