You would need to speak to the postgraduate chair or head of department about this. If the new topic is still somehow related to the original supervisor, the department might assign a second supervisor for you- which you can keep the original supervisor and have a second supervisor who is more familiar with your topic.
In any case, most university does not "question" when a student want to change his/her supervisor. In your case, it's perfectly normal because your supervisor might not have sufficient knowledge in the new topic.
Hope you can sort this out soon!