Yeah. I triggered it by running wuauclt.exe /updatenow but it failed after downloading around 6GB data in $Windows-BT and 2GB data in $Windows-WS
Found the solution. Writing this post from win 10. Drivers update going on.
Copy the below file to a folder in separate partition (let's say D:\win10)
C:\$Windows.~BT\Sources\install.esd to D:\win10
*i.imgur.com/NDZM0HZ.png
Download ESD decrypter
ESD decrypter (Alternate download link: ESD decrypter)
(Source Instructions: Create a bootable ISO file using ESD Image for - Microsoft Community)
Extract the content to win10, it should look like this
*i.imgur.com/haP1yvw.png
Run the decrypt.cmd file as admin
*i.imgur.com/88ilgtZ.png
Enter 1 as the option to create bootable iso
*i.imgur.com/wFn1eAM.png
You will see the iso once the process is completed. ISO size is 3.71 GB (64 bit edition).
*i.imgur.com/eOyqmhF.png
Mount it in win 8/8.1 as virtual drive
*i.imgur.com/bDLSRPi.png
Go inside the virtual drive and run the setup to initiate win 10 install
*i.imgur.com/hh63BmB.png
Uncheck the box and select not right now (will save you time)
*i.imgur.com/ZwJu3vN.png
Accept
*i.imgur.com/Mymp1hU.png
Click on Install. By default win 10 install will do an upgrade to win 8/8.1. So just continue installing.
*i.imgur.com/ZmsMo3s.png
Your system will restart and begin updating. A couple more restarts will follow. Tip: Unplug your internet connection for a speedy install. You can update later once your win 10 is installed.
*i.imgur.com/8Orliku.jpg
Logon screen
*i.imgur.com/qaKKEjQ.jpg
*i.imgur.com/SL4vOAD.png
Reconnect your internet connection. Your video driver and other stuff will begin updating in background. You can download drivers directly if you wish.
Few apps will be incompatible to some extent. Update should fix that.
Now you have a Windows 10 iso too. Use it to install on other computers.