After 160 hours on CK2, I still got noob questions : wrong type of holding in desmene

Hi,

As I'm planning to recreate the crown of England, destroyed by the damn Scots (now Emperors of Brittania), I needed to take over a duchy held by a prince-bishop.

As I couldn't find a claimant, I fabricated a claim on its duchy capital, and quickly won the war. Now I'm the countess of a province with a bishopric that I own, a town (with a mayor) and a castle (held by a Baron).

I don't want the land, I'm already above my limit. But I don't want a bishop to rule : I want a regular count with a regular castle.

How do I do that ?