How to upgrade R to the latest version on Ubuntu 14.04

Follow this instruction:

https://www.digitalocean.com/community/tutorials/how-to-set-up-r-on-ubuntu-14-04

but first you need to add the key to the Ubuntu keyserver:

sudo gpg --keyserver-options http-proxy=http://YOURPROXYSERVER --keyserver keyserver.ubuntu.com --recv-key E084DAB9

where YOURPROXYSERVER is the address of your proxy server.