Just installed NIB on STEAM and it worked fine for a while. Then it asked me to install some type of "upgrade" so I did.

But since then it doesn't work well, it's not smooth panning and image "stagers"( tiny freezes) in regular intervals.

So I re-installed it, but somehow it kept all my settings (?), so it was not completely re-installed from scratch, and I have the same problem. It didn't ask me the serial number, so I believe it just re-installed it from STEAM "memory" on my PC.

How do I re-install it completely from scratch to STEAM ? Thanks

All your settings are in the Evolution folder in wherever your "My Documents" are stored. That does not go away when you uninstall.

But first, make sure you are NOT using Microsoft One Drive, at all. That can play havoc with performance and give exactly that kind of behavior.

STEAM bases everything on your user account, so doesn't need serial numbers.

EDIT- OK I checked, I don't have Microsoft One Drive on my PC. So what now ?

Go to the forum linked below, register, and post about the problem. Be detailed about your computer specs (brand, processor, video card brand and model, etc.). Note what controller you are using for the sim.
Be as descriptive as you can about the problem, especially anything about the unknown "upgrade" - which probably wasn't an upgrade from Real Flight, but something else on your PC.

https://forums.realflight.com/index.php

Many very experienced users there, and while not an official support channel for Real Flight, it is occasionally visited by their staff who will also help with specific issues.

Wil do, thanks