Import packages
library(plyr) # loading plyr for mutate function
Check current reference level of Species
levels(iris$Species) # current reference level is setosa
## [1] "setosa" "versicolor" "virginica"
Change reference level from setosa to versicolor
iris$Species <- relevel(iris$Species, ref = "versicolor")
levels(iris$Species) # reference level changed to versicolor
## [1] "versicolor" "setosa" "virginica"