Changing current supervisor
You should check your department's policy to see if it requires you to find a replacement research supervisor in a certain time frame or following a certain procedure.  Then you should inform the professor.  

Your success as a graduate student will depend on starting your doctoral research as quickly as possible (assuming you are not a masters student).  If you are going to change your plans, the sooner you do it, the less harm is done to your own success and that of your department.