Is it the change that you get from the cashier when you pay with a banknote, or just some small coins you have in your wallet, or both of them?
It's the change you get back from a cashier. The small coins in your pocket/wallet would be "мелочь" (which literally means something like "small stuff").
So "change" is uncountable?
Yes, in that meaning it is. Just like "сдача" in Russian.