Switching governments
The government may be changed by entering the manage menu on the left hand side while logged into the game. Within this menu is a radio button section with a range of governments. Simply select one of the government types and click on the button Start Revolution.
Effects Of Changing Government
If your current government is Monarchy, then there is no penalty for changing to another form. When changing from any other type of government to another, there are direct cash, military, technological, building, and bonus losses incurred due to your country's instability. The manage page will tell you roughly how much money, buildings and technology you will lose.