From the Glossary under Regime Change:
REGIME CHANGE - This occurs in an Empire during a coronation (I1) or if you wage a victorious vote (F6), campaign (F9), or Civil or Religious War against that Empire.
Own Empire. Except in campaigns (see next bullet), if the Empire is in your own Tableau, flip it to its other side. Discard any Queens and Vassals but retain all Tokens. If it was a Vassal, move it to your outermost Tableau position as a Republic, rightmost or leftmost depending on if it is east or west. Successfully attacking your own Empire is called a strawman regime change, and is the only way to create Republics.
If I Strawman my own republic does it flip back to kingdom again?