Updating Windows 10

HIYAS
What's the best way of updating Windows 10 in case there's already configured instance of earlier build? I.e. to not touch existing user accounts etc. Should I need to launch the setup from within the Windows or boot from DVD? Which option shall I choose, update or custom install? I do have also Windows 8.1 instance and don't want it to update this version. Will the update way ask me which partition to update?
And finally how much free space do I need for update?
Thanks

You're basically wasting your time trying to do this before the 29th. MS has turned off the activation servers for Win10 and has also disabled all the Insider Preview keys. While you could install, you could not activate -- so that is basically a waste of time.

In order to run Win10 without buying a license, you would have to upgrade your Win8.1 instance to Win10. If you try to clean install Win10 from an ISO after the 29th, you will need a purchased key in order to activate it, or you would have to join the Windows Insider program.

What I would recommend is installing Macrium Reflect free in Win8.1 and just before you want to do the Win10 upgrade, do a full image backup of Win8.1 to an external drive. That way, if the upgrade goes badly, or you simply don't want to keep using Win10, you will have a backup you could use to restore Win8.1.

My experience is different. Having gone through the upgrade path from Windows 10 builds 10130>10158>10159>10166>10240
I had a working but slightly battered installation of Windows 10 build 10240. For instance it takes a long time to boot up, and a few other niggles.

I had made ESD>ISO copies of all builds except 10130, which was the ISO download itself.

I decided to clean install from the 10240 ESD the next day, before I knew that the activation servers had been downed. So I wiped it, having limited space for another installation.

The only ISO install that activated was 10130 but I couldn't get it to upgrade.

So I installed 10166 on the 16th, and hoped it would update online, which it did the next day (17th), having made sure I regularly checked for updates, (and looking for TH1) and installed them. However the 10240 upgrade was not activated, and oddly, had the full build number of 10.0.10240.16389.

The various attempts to activate failed with the -3V66T product key. I did not intend to go down the KMS route and lose the retail channel license if it ever arrived.

Exploring it further, the initial watermark to activate had disappeared, I could personalize the desktop, but still remained unactivated. Something I had not noticed at first was the absence of tiles on Start! I decided to reset, to hopefully restore full function and fell asleep on the 18th.

The next day (19th), on restarting the PC it had hibernated, and after flashing through the login screen, it wen into Setup with Hi, Getting things ready etc. Finally, after "It's taking a little bit longer than usual..." it rebooted and upon opening System, it was activated., and it seems to be working OK.

Thank yas, in short if there will appear a newer build before official sale date, will the core update fully via WU or do I have reinstall from ISO? I don't like "Windows.old" folder because it sounds like the ISO setup will do a clean install.