Update list B when an update occurs in list A using event receiver

I have a Last A as Master list and List B as Slave List. Slave list is kind of back up list for List A. Whenever we add item to List A, the same item is added to List B, I achieved this action by creating an event receiver. Now I want an update functionality. Whenever an item in List A is updated the corresponding item in List B should be updated.

I walked around various methods but none is clear. Kindly help me...!