Installing W10 in a HD partition

Hello, I have a Windows 8.1 system (64 bit) and I "qualified" for the Windows 10 free upgrade, which I could already do.

(My system is in French, sorry about that).)

What I'd like to do is to have both systems in the same computer, I know how to shrink my HD in order to have enough room for a system.

The problem is I don't know how to install W10, the official released version, in a partition while keeping my original system, Windows 8.1, in another partition of my HD. I want to be able to choose between systems when I boot my computer.

Thank you

The problem is I don't know how to install W10, the official released version, in a partition while keeping my original system, Windows 8.1, in another partition of my HD.

Win10 has to be installed as an upgrade to an existing installation. What you're trying to do is do a clean install to a different partition. While you CAN do that, it is not an upgrade, will not carry over anything from your existing install, and most important, will NOT activate. There is no capability to then activate Win10 from an existing Win7/8/8.1 key.

I want to be able to choose between systems when I boot my computer.

You can't do that with the current Win10 upgrade. When you do the upgrade, it inherits the license from your current Win7/8/8.1 installation. This means that you can not legally then go back and continue to use that Win 7/8/8.1 installation -- unless you remove Win10 and reinstall the prior OS.

Mark Phelps said:

Win10 has to be installed as an upgrade to an existing installation. What you're trying to do is do a clean install to a different partition. While you CAN do that, it is not an upgrade...

So, if I understood correctly, what I intend to do is only possible if I paid for the OS?
Thank You