I have an ESTA for my visit last Sept to NY and I know it is valid for 2years. I am going to Las Vegas in May, do I have to update the address at which I will be staying?
No, you don't have to do so.
The official line, from the CBP FAQ page on the VWP and ESTA:
Can prospective travelers apply for an ESTA without specific travel plans if they want to be able to travel to the United States later on short notice?
Yes. VWP travelers are not required to have specific plans to travel to the United States before they apply for an ESTA. If a traveler’s destination in the United States is unknown when he or she completes the ESTA application, the traveler should enter “Unknown.” Travelers may update this information when their plans are finalized, but they will not be required to update their destination addresses or itineraries should they change after their ESTA application has been approved. DHS recommends that ESTA applications be submitted as early as possible, as soon as, or even before travel is planned. ESTA will accept applications from last-minute and emergency travelers – those VWP travelers who arrive at the airport without an approved ESTA.