It's like how animals don't "evolve", rather then ones who DONT change simply die.
Untrue - some (many) mutations are bad, and the ones that change are the ones that die.
Evolution occurs when a mutation is a) beneficial and b) significant enough that it allows an animal to survive longer than those without that mutation. Being alive longer means they breed more. Breeding generally passes on the mutation.
It really is quite random, and there is no guarantee that the animals that change are better. Indeed for an animal that's particularly well suited to an environment that hasn't changed in a long time there could be very little room for evolution to occur. Certain types of fish (like gar and bowfin) for example have survived for tens of millions of years with relatively minor evolutionary changes.