sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv 7F0CEB10
echo 'deb http://downloads-distro.mongodb.org/repo/ubuntu-upstart dist 10gen' | sudo tee /etc/apt/sources.list.d/mongodb.list
sudo apt-get update
sudo apt-get install mongodb-org

RESOURCE

https://www.digitalocean.com/community/questions/how-to-upgrade-mongodb-2-4-to-2-6-on-ubuntu-14-04