im in mid game and i want to reform my goverment but i cant change it to the kind of goverment but i cant do it becuse of the current ethics, how do i change them?

goverment policy is F6 and you can ajust them as needed

and you need a spare slot which can happen during play but in most cases you unpick one to get something else i.e, if I drop safe haven then I could use the space to take police state instead

note clicking an Authority option like Corporate (in my caes) will move all the options that allows to the top of the list

be aware that making any change will lock you into it and many races will change how much they like | hate you when you do this

First, you need a faction that embraces the ethic you want to be in your empire. The easiest way to do that is to conquer a planet from a rival empire that has those ethics, then give the conquered population residence or citizen rights, and the faction will pop into existence almost immediately. Now select the faction and choose the "embrace faction" option.