Is it available for all or for some specific regions because i couldn't find any new build in windows update option
Mine is downloading fine. It is not in Windows Update section but in your PC Settings. Try to check for new build in under the 'Preview Build' area.
First filename is WindowsStoreSetupBox.exe, ESD is 9860.0.141008-2044.fbl_release_CLIENTPRO_RET_x64fre_en-us.esd This is for the normal Preview. Made a copy, just in case.
Impossible to tell from that post, but I'll be curious to hear if this update method is really doing the same thing behind-the-scenes as if you had the ISO and just did an upgrade.... If so, I'd rather use the ISO, even if it's a little larger DL (which it is).
From the Windows blog, I quote, "The download will range between 2GB and 2.74GB depending on CPU architecture and language."
Microsoft claims to have made 7000 improvements on this new build. That is lot of improvements to me. I hope the Ultra-high 'DPI' resolution scaling is among one of them.