Upgrading R in Ubuntu 14 topic

I'm using R version 3.0.2 and I'd like to upgrade it to version 3.1.2 on Ubuntu 14.04 LTS.

I've tried to do all this:

sudo gedit /etc/apt/sources.list
# deb http://cran.es.r-project.org/bin/linux/ubuntu trusty/
gpg --keyserver keyserver.ubuntu.com --recv-key E084DAB9
gpg -a --export E084DAB9 | sudo apt-key add -
sudo apt-get update
sudo apt-get upgrade

However, I cannot upgrade the program:

R.Version()
$version.string
[1] "R version 3.0.2 (2013-09-25)"

Could anyone help me?