Master's thesis - I want to change subject and/or supervisor

I still want to change subject and supervisor before the deadline for registration in SisA

You first notify your current supervisor. To enter a new subject in SisA, find more information here: Manual: Change or cancel your dissertation request.

I still want to change subject and supervisor after the deadline for registration in SisA

If you are enrolled in the Master of Political Science, you have to indicate a top 3 (choice of supervisor). You will then be assigned to a supervisor by the programme manager. After this, you cannot simply change your supervisor. You will have to motivate this thoroughly and there is no guarantee that you will be able to change your supervisor after you have been assigned a supervisor.