How does the top come apart from base cabinet?

It is probably glued along the top edge of the cabinet. Usually a bead of liquid nails or similar product is use in spots. If you start at a corner and gently pry up between the cabinet and the top it will pop and then work your way down. Sometimes, there are corner braces in the cabinet with screws up into the top. You will just have to look and see. In the front corner of the cabinet.