Where do you see you system level?

[Imortalin]

I see it mentions in some techs but how do i see what level my system is?

[The-Cat-o-Nine-Tales]

There should be a panel at the top left of the System View, near the FIDSI display.

[Mc]

Also in empire view (list of systems) there is a number on each system. But it is labelled differently, so it took me some time to understand it is actually system level. This list is useful if you want to find out which systems you did not upgrade yet.

To upgrade you must first go to economy(?) view - the one with trade companies, and on the right side you must define which resources you will use for upgrading the system (for this to unlock you need enough technologies from the right section of the science wheel). Once defined you build it on the system like anything else.