The R version in (k)ubuntu 14.04 doesn’t upgrade automatically, and you have to add a repository manually. This has worked for me:

• Edit the file /etc/apt/souces.list as a superuser and add the repository http://cran.rstudio.com/bin/linux/ubuntu trusty/. This procedure can also be done executing one of these lines: trusty for the 14.04 and xenial for 16.04.

  sudo sh -c 'echo "deb http://cran.rstudio.com/bin/linux/ubuntu trusty/" >> /etc/apt/sources.list'
  sudo sh -c 'echo "deb http://cran.rstudio.com/bin/linux/ubuntu xenial/" >> /etc/apt/sources.list'

• Add the public key to the system and to the apt:

  gpg --keyserver keyserver.ubuntu.com --recv-key E084DAB9
  gpg -a --export E084DAB9 | sudo apt-key add -

• Update the  software list from the repositories:

  sudo apt-get update

• Either upgrade the software

  sudo apt-get upgrade

  or install R:

  sudo apt-get -y install r-base