With base R, use
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.
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