If you installed R from the Ubuntu repository with
sudo apt-get install r-base
you most likely got an out of date version. In February 2018, that method still gave me R version 3.2.3 (2015-12-10). To get the latest versions of R and its packages, you need to add CRAN to the apt-get repositories. Do this with the code below. Enter one line at a time. Cut and paste to prevent errors.
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys E298A3A825C0D65DFD57CBB651716619E084DAB9
sudo add-apt-repository 'deb [arch=amd64,i386] https://cran.rstudio.com/bin/linux/ubuntu xenial/'
sudo apt-get update
Now if you open Synaptic Package Manager and search for r-base, you should see the installed versions and the latest versions of several packages (e.g. r-base, r-base-core, r-base-core-dbg, r-base-dev, r-base.html). Mark the packages for upgrade and click on apply. This will most likely result in a major update of R so that the next time you run R only the base packages will be available. You will have to re-install any additional R packages that you use to match the updated version of R.