[r] adding x and y axis labels in ggplot2

How do I change the x and y labels on this graph please?

library(Sleuth2)
library(ggplot2)
discharge<-ex1221new$Discharge
area<-ex1221new$Area
nitrogen<-ex1221new$NO3
p <- ggplot(ex1221new, aes(discharge, area), main="Point")
p + geom_point(aes(size= nitrogen)) + 
    scale_area() + 
    opts(title = expression("Weighted Scatterplot of Watershed Area vs. Discharge and Nitrogen Levels (PPM)"), 
         subtitle="n=41")

since the data ex1221new was not given, so I have created a dummy data and added it to a data frame. Also, the question which was asked has few changes in codes like then ggplot package has deprecated the use of

"scale_area()" and nows uses scale_size_area()
"opts()" has changed to theme()

In my answer,I have stored the plot in mygraph variable and then I have used

mygraph$labels$x="Discharge of materials" #changes x axis title
       mygraph$labels$y="Area Affected" # changes y axis title

And the work is done. Below is the complete answer.

install.packages("Sleuth2")
library(Sleuth2)
library(ggplot2)

ex1221new<-data.frame(Discharge<-c(100:109),Area<-c(120:129),NO3<-seq(2,5,length.out = 10))
discharge<-ex1221new$Discharge
area<-ex1221new$Area
nitrogen<-ex1221new$NO3
p <- ggplot(ex1221new, aes(discharge, area), main="Point")
mygraph<-p + geom_point(aes(size= nitrogen)) + 
  scale_size_area() + ggtitle("Weighted Scatterplot of Watershed Area vs. Discharge and Nitrogen Levels (PPM)")+
theme(
 plot.title =  element_text(color="Blue", size=30, hjust = 0.5), 

 # change the styling of both the axis simultaneously from this-
 axis.title = element_text(color = "Green", size = 20, family="Courier",)
 

   # you can change the  axis title from the code below
   mygraph$labels$x="Discharge of materials" #changes x axis title
   mygraph$labels$y="Area Affected" # changes y axis title
   mygraph



   

Also, you can change the labels title from the same formula used above -

mygraph$labels$size= "N2" #size contains the nitrogen level