One thing that I discover yesterday... Get rid of any VPN client like Cisco VPN, etc before upgrading. Else can wreck havoc with the internet connectivity.

Other than that, smoothest upgrade experience. 

If you change your mind, can go back to Win 8.1 with a couple of click. Your machine will be return back to Win 8.1 perfectly. 

Sorry, don't understand your sentence... On Win 10, revert back to Win 10?

for me, i make bootable using rufus into usb then run setup.exe from usb to upgrade 

You need the tool to create bootable Windows10 thumbdrive.
https://www.microsoft.com/en-us/software-download/windows10

After execute it The ISO creator will download the Windows 10 installation file and copy to your thumbdrive.

Rufus is not needed, unless you already have to ISO.
Because the Windows 10 media creation tool will download Windows 10 from scratch iirc

wrong windows version. check your windows version (it's either pro, home, home single language, x86 or x64) and download the correct iso.
if you have upgraded before (run setup.exe in windows), just skip places asking you for key.

Run setup.exe found on the ISO from inside Windows 8.1 first to upgrade, this will activate Windows 10 and tie it to your machine.

Reset/repair/clean install ONLY AFTER upgrading ....

mount the downloaded iso to ur hdd and run the setup.exe file

yeah the windows 10 installation files u downloaded in iso form, u extract it to ur desktop or whatever. Then run the setup and upgrade

the upgrade path has been highlighted many times in this thread. i have no clue why people are still getting it all wrong lol

1. you need to upgrade the activated os first. connect the computer with activated os to the internet, download the iso with the correct windows version for the activated os with media creation tool. extract the iso contents in a folder or just create a bootable usb out of the iso. run the setup.exe in the folder or bootable usb in normal windows environment (not booting it from usb) to do upgrade (the computer must be connected to the internet during the upgrade);
2. once you have completed the upgrade, connect to internet again to make sure the new w10 is activated. that's all for the upgrade.
3. if you wanna do clean install, do step 1 and 2 above first, then boot the computer with the bootable usb instead of windows. you can wipe the os drive clean, and then proceed. when asked for key, skip it and proceed. after clean install, connect the computer with new w10 to the internet and it will be activated automatically. no key is required.

note that for the free upgrade offer, you only need to run step 1 and 2 once (the upgrade), then you can do clean installs as many times as you like, without w10 key, for the lifetime of the device.[SIZE=7]
i think your windows version is single language x64 bit.
wrong. read my reply above.

Yes. This is made possible because the W10 activation is tied to your existing hardware configuration (which is kept by Microsoft), so you can clean install on the same device (with minor upgrades) as many times as you like.