Question

how would it be possible to change the phenotype of an individual

Answer 1

There are many ways! An individual's phenotype is composed of its observable characteristics or traits, and unlike the underlying sequences of DNA, many of those can be changed. Even something as simple as dying hair is changing a phenotype: someone's genes could say "blonde", but the phenotype that they display (as an interaction between their underlying genes and the environment, meaning the hair dye used) could change to brunette. Genes can completely control changes, as well, but this is an example where that isn't the case.