Although this is Linux Blog I have decided to write about my upgrade experience from Windows 8.1 to Windows 10.
My LenovoG50-45 came with pre-installed Windows 8.1. I had disabled Automatic Update on Windows 8.1 and use to update once in a while. When MS decided to give free upgrade to Windows 10 I went through the procedure which asked for applying all pending updates to Windows 8.1 to see Get Windows 10 application. When I clicked on the application it asked to wait for my turn. Becoming restless I searched for how to get immediate upgrade and enabled something which did not work. Finally I got permission to download on Get Windows 10 application and started download of about 2.8 GB. After download got finished something happened during preparation to install and it started the download of 2.8 GB once again.
Getting frustrated I decided to re-install Windows 8.1 through one-key recovery. While booting in re-installed Windows 8.1 MS asked whether I was interested in upgrade to Windows 10 and I said yes to it.
Windows 8.1 re-install damaged Grub2 which I corrected using Boot-repair.
After couple of updates to Windows 8.1 (not all) Windows 10 download appeared on Windows update and I finished the download in 6 hrs (I have 1 Mbps connection).
After finishing download it changed to "preparing to install" and I had to wait patiently for this activity since Windows extracts the 2.8 GB file in the background.
Finally it was ready to upgrade after accepting EULA. The upgrade took lot of time and there were three reboots. On the last boot Windows asked several questions and the best one was to go for express install which I declined and chose "Custom".
After booting to the Desktop Windows 10 did not allow to disable automatic updates but I searched in Wi-Fi Settings to declare my connection "metered" which prevented the automatic update.
Fortunately Windows 10 upgrade did not touch grub2 and I could boot to Ubuntu 14.04.3.
From Ubuntu 14.04.3 Grub2 I could not boot Ubuntu 15.10 using systemd and had to use upstart option. I used Boot-repair once again on installed Ubuntu 15.10 to re-install Grub2.
Finally I have Windows 10/Ubuntu 14.04.3 LTS/Ubuntu 15.10/android-x86 multi-boot successfully working on my laptop.