If us humans were supposed to have evolved from primates, apes etc how come they still exist today?
Ditto to Tao's answer. Also, the species from which modern humans evolved do not exist today. Primates are a huge group ranging from lemurs to baboons to chimps to us. It's like saying "If horses exist, why aren't Zebras extinct?" Well, because multiple species can co-exist on earth. Diversity of environment lends itself to diversity of species.
Second, we did not evolve directly from any species of living ape. Our evolution quite clearly began about 4 million years ago with australopithecines, which were a number of species of upright walking, but still quite chimp-like, apes. I can recommend some good, basic reading on human evolution if you like.
If something "evolves" all previous is superceded and the new replaces?
No.
Also why is no skeletal evidence seldom found to show a creature's evolution?
There is plenty of skeletal evidence in the case of humans. It is really easy to see the gradual shift to modern human anatomy when presented with skeletons starting with Australopithecines, then onto Homo habilis, and then Homo erectus, and then the early and late Archaics, and then modern Homo sapiens. We may not have discovered all the species of humans yet, but there are no "missing links" in the sense of large gaps. We already have enough to have a clear timeline and evolution toward upright walking, larger body size, larger brain, and more complex culture and social structure.
A scientist would say "now this creature has evolved from that" but not much skeletal evidence is found to show the gradual evolution stages, as in a skeleton with legs from when they were 1 foot tall to being 5 foot tall.
Mutations can have very large effects. Evolution is based on the mostly random occurrence of genetic mutation, and the environment's selective process on those mutations. A mutation can have rather profound effects on a creature at times. If it is favorable in the environment and is a mutation found in the sex cells (gametes), then it may provide that creature with more offspring over its lifetime (and most importantly, more grand-offspring) and over time this may become a new species, either exploiting new niches within an environment, or out-competing other species in the environment, or allowing it to radiate to new environments.
A basic course or textbook in evolutionary biology or biological anthropology will explain all this. I can recommend a few good ones, if you like.