You want to change the order of levels in a factor.

The level order can be specified explicitly by passing the factor to factor() and specifying levels. In this example, we’ll create a factor that initially has the wrong ordering:

# By default, levels are ordered alphabetically
sizes <- factor(c("small", "large", "large", "small", "medium"))
sizes

small  large  large  small  medium
Levels: large medium small

# Change the order of levels
sizes <- factor(sizes, levels = c("small", "medium", "large"))
sizes

small  large  large  small  medium
Levels: small medium large

The order can also be specified with levels when the factor is first created.