Scenario: you just broke your phone and you need to pick between the cheaper iPhone 5 and the expensive iPhone 6 and your income has increased this week, which one will you choose?
Answer: your demand for the cheaper iPhone will change because if you can afford the newer version you will buy that one instead.
Scenario: This winter UGG boots are at an all time high people are spending big time money to get these simple boots. But next winter you see that no one is wearing them, why?
Answer: because demand changes as trends fade out so at one point they're demand was super high and now the demand is low because no one is buying them.
Scenario: you're having a party and you need an appetizer so you think of chips and salsa and they are always sold separately but you need both, will your demand change?
Answer: yes your demand will be higher because you can't have chips without salsa!