It is said that the virii of commoncold mutates everytime, as it invades into a host. How the mutation occur?

Pls explain me clearly. i dont know if those viruses mutate EVERYTIME, but contained by general, viral nucleic tart molecules are very susceptible and are expected to undergo mutation. this in some measure caused by the simplicity of the molecules itself. they dont own proof reading activity( like we human and complex organisms have) and they lack stabilisation proteins to bind to the molecule hence the frequency of the mutation.
selective evolution and mutation within order to ensure the survival of an living organisum, as in good health as adaptation thru breeding cause by selective adaptastion to medications, during it's spree to become the perfect organisum that will snuff all of it's symbiotic hosts that it comes surrounded by contact with.