Principle of Duality I would like to know how to alter/add NOTs when applying Duality principle. 

Suppose I have P = XY(X+Y) +  NOT(Y), how to find its dual?  

My book says that while applying Duality Principle to a relation: 


Change all 0s to 1s 
Change all ORs to ANDs
Change all ANDs to ORs


But it doesn't say anything about NOTs.
In the dual boolean algebra, NOT is the same NOT operation we have in the original boolean algebra.

Your book does not say anything to handle NOT because there's nothing that needs to be done. Simply leave the NOTs unaffected in your formula.