Non Destructive Rebuild of Win8.1

My galfriend's laptop is having problems with its OS.
I tried most everything to get it to run right, no satisfaction.

The next step is to try an ND Reinstall.
I have done it a few times over the years for my WIN7 PC.
Win8 appears to be done differently.

Correct or incorrect?
1.
The ND Rebuild cannot be done from the Win8.1 install disc.
It must be done from an upgrade disc, or similar.

2.
Can I download the upgrade ISO or what ever format it comes in to my Win7 PC?
Or, does it need to be downloaded to a Win8 machine?

I have downloaded a couple of tutorials for doing it.
Win8 seems to be much more complicated than doing it for Win7.

I may have a few more questions as I procede with the operation.

Just download the current windows 8.1 ISO
Mount the ISO within the current faulty OS (the one you would like to repair)
and just run setup.exe from that ISO

it will ask you if you want to keep current personal files and settings > yes

You will be overwriting the current OS with a clean version of the OS..
the old version of the OS will be placed in a folder /windows.old

so anything you may have or need from the OLD OS that was not carried over into the New OS > can be found in that windows.old folder