I was just about to update R a while ago when I thought to myself that there must be a way to do this inside of R (RStudio, I mean). A quick Google search brought me to the installr package. Very nice, but I use a Mac. Hmmm...
A bit more searching and I found Andrea Cirillo's updateR package, which was made for OS X, fantastic. I tried it out, and although it worked great, I still had to leave RStudio to check to see if the latest version installed. I really liked Andrea's package, so I thought I would suggest a few changes to him.
Basically, now the function prints out a few more informative messages and makes it unnecessary to have to leave RStudio. I've used it since then and all works great, so next time you need to update R on your Mac, you can do it from inside Rstudio ;-)