explain with example how to change a positive sign while the two negative sign of products.

Actually we are simplifying when we write a double negative as a positive. One way to see this is to recognize that one interpretation of the negative is to reverse a number's direction from 0 on the number line. If the number is negative, reversing its sign makes it positive. 
Ex1: -(-2)=(-1)(-1)(2)=2
Ex2: -(-(-2))=(-1)(-1)(-1)(2)=[(-1)(-1)](-1)(2)=(-1)(2)=-2

In fact the overall sign of a product is positive if the number of negative signs is even and negative if that number is odd.