You can use the theme layer to center align the title. Below is the command solves your problem:
ggplot(data=iris,aes(x=Sepal.Length,y=Petal.Length))+geom_point()+ggtitle("Sepal.Length vs Petal.Length")+theme(plot.title = element_text(hjust = 0.5))
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You can also change other parameters of the title such as font color and font style:
ggplot(data=iris,aes(x=Sepal.Length,y=Petal.Length))+geom_point()+ggtitle("Sepal.Length vs Petal.Length")+theme(plot.title = element_text(hjust = 0.5,colour = "blue",face = "bold.italic"))