Domestic Government interface
The Domestic Government interface screen is found on the sixth tab of the Domestic Management interface. At the top of the screen, it shows your government form, and by clicking on the button, it will allow to change your government
In the middle, are the boxes showing your country's national ideas. If your government tech improves to allow a new idea, this is where you choose it.
At the bottom, there are your domestic policy sliders, where you can adjust your politics. If your government form limits your sliders, this will show as a red arrow like in the top slider in the image. You can go over the limit, but for each point over the limit you get +1.0 revolt risk.