You want to change the text of labels in a legend.

Set the labels in the scale (Figure 10-13, left):

library(gcookbook) # For the data set

# The base plot
p <- ggplot(PlantGrowth, aes(x=group, y=weight, fill=group)) + geom_boxplot()

# Change the legend labels
p + scale_fill_discrete(labels=c("Control", "Treatment 1", "Treatment 2"))

Figure 10-13. Left: manually specified legend labels with the default discrete scale; right: manually specified labels with a different scale

Note that the labels on the x-axis did not change. To do that, you would have to set the labels of scale_x_discrete() (Changing the Text of Axis Labels), or change the data to have different factor level names (Changing the Names of Factor Levels).