pointless summary of earlier presented information
mutations are basically changes that take place in the genetic information.
Every organism is subject to mutation, because DNA is not 100% stable. There are some mechanism that repair damaged DNA, but once again, this system is not 100% perfect.
Evolution acts on genetic variation. And since everything that contains DNA or RNA mutates, they also are under the influence of evolution. A virus contains DNA or RNA. Viruses don't even have repair mechanisms (as far as i know of), therefore they are certainly subject to mutation. Hence they can evolve.