Over a period of two generations, the frequency of green dung beetles in a population shifts from 75% to 71% while the frequency of brown dung beetles within this population shifts from 25% to 29%. what is this is an example of?

This is an example of microevolution.

Microevolution represents a small change in the genetic structure of the population. These changes manifest through the change in the allele frequencies of genes like in the case of dung beetles.
The causes of microevolution are evolutionary mechanisms, natural selection, genetic drift, mutations and gene flow.