On a Windows 8.1 system, you should be able to just right-click the ISO file to mount it.
Then you will be able to access its contents and copy them to the USB drive you've prepared.
To boot from the USB stick, ensure that your BIOS is set to boot from the USB stick, and your system should then boot into the Windows 10 setup routine.
The following are some photos of the Windows 10 installation procedure. It looks familiar.
And here are some shots of Windows 10 in action. The overall look is flatter, with no discernible borders around windows, and, of course, the Start menu is back and incorporates elements of the Start Screen.
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