how do i manipulate factor levels of a data frame in r?

With base R, use factor():

Vitamin_C$size = factor(
  Vitamin_C$size,
  levels = c("(1,20]", "(20, 35]"),
  labels = c("Reduced", "Normal")
)

You could also directly modify the levels (make sure they start in the order you think they are!)

levels(Vitamin_C$size) = c("Reduced", "Normal")

As mentioned in comments, if you are creating this factor with cut, you can specify the labels you want directly in the cut command, which is best of all.