Mutation in a virus is caused due to a change in its :

A mutation is defined as a change in the genetic material (DNA or RNA) of an organism or virus. Viruses are microscopic, acellular entities that lack true cell structures; therefore, changes in cell size are not applicable. Instead, mutations occur during the replication process when the viral genome undergoes copying errors. These alterations in the genetic material can lead to changes in the function of viral proteins, potentially resulting in new viral serotypes or altered pathogenicity [1]. While DNA viruses have mutation rates similar to eukaryotic cells, RNA viruses exhibit much higher mutation rates, often yielding one mutation per genome copy [1]. These genetic variations are the primary source of viral evolution, allowing viruses to adapt to new environments, escape immune responses, or develop drug resistance.

Sources

1. [1] https://www.ncbi.nlm.nih.gov/books/NBK8439/