Here is a link for the Media Creation Tool. http://mab.to/QbhB7jF8K
It's in a .zip file so you'll have to extract it first.
A Menu will ask you what to do. Select Create installation media for another PC. Click next.
Then select you language, then choose Windows 10 Home or Pro from the menu, then select whether your machine is 32 Bit or 64 Bit. If your not sure which one you have goto Control Panel and clck System.
The System type will show 64 Bit if you have it or it will show 32Bit.
Once your version is selected click next. Then select .iso click next. Then a menu appears asking you to select where you want the media downloaded to select your destination and click next and the download begins. Depending on the speed of you internet and computer the Download could take 10 minutes to a couple of hours as the file is 3.26GB in size. After it's downloaded do not extract the .iso. You can burn it to a DVD Disk or you can extract it using a program called Rufus. Here is a link for that http://www.filehorse.com/download-rufus/ Rufus is only 843kb in size and there is no set up just click and below the 1 Pass select .iso from the menu and the box at the right to select from a menu where your .iso file image is after that's selected click next and it will install it to a flash drive.
Once that is done from your windows 8.1 insert your DVD disk or Flash Drive and click set up do not click Custom or it will do a clean install and wipe your drive clean. After set up starts just sit back and wait. It could take from 12 minutes up to an hour depending on the speed of your system.
It will keep all your file, settings and programs the same so whatever is installed will still be in the same spot. It creates a Windows.old folder next to the Windows Folder. Windows.old is just the files of your Windows 8.1 incase you ever decide you don't want Windows 10 then it can be Rolled Back to Windows 8.1.
If you wish to Roll Back it has to be done within 30 Days or the Roll Back Feature will no longer work. Then a clean install is required to go back to what you had previously unless you have a backed up image of your 8.1 then you'll have no problems.
Sorry about the long post. I figured I'd better explain it if you don't know how. Also this post might be helpful for others that aren't up to snuff on how to do it. It's certainly better than paying a retailer $100 or more to do it when you can do it yourself for free. Hope this helps.