Worked! 🙂
So I have iMac MD094 (2,9 Ghz, 8Gb, 1Tb HDD and 500 GB Samsung SSD EVO 850 which is used as system disk for both Mac OS X and Windows with 2 partitions)
So this is my working pattern to upgrade your Windows 8.1 or 7:
1. Reserve your copy
http://www.microsoft.com/en-us/windows/windows-10-upgrade
2. Wait until notification about Windows 10 upgrade and install Windows 10.
If you do not want to wait follow these steps:
a) Delete all files and folders here C:\Windows\SoftwareDistribution\Download
b) open Windows Update window from Control Panel
c) run command "wuauclt.exe /updatenow" to force upgrade through WUA (Windows Update Agent)
d) after executing command you will see how Windows 10 is being downloaded in the Windows Update window.
e) you will be asked to install Windows 10, computer will reboot many times during installation
3. Go to Mac OS, open Bootcamp Assistant, choose only one item to download support software, specify flash drive as location
4. Go to Windows, go to flash drive, go to BootCamp folder, run setup.exe, bootcamp will be updated.
5. All works fine, keyboard and mouse are fully supported in Windows 10!
6. Go to NVidia Control Center, install driver for Windows 10
7. Enjoy! 🙂