Per site ( https://nginx.org/en/linux_packages.html ):

For Ubuntu replace codename with Ubuntu distribution codename, and append the following to the end of the /etc/apt/sources.list file:

deb http://nginx.org/packages/mainline/ubuntu/ codename nginx
deb-src http://nginx.org/packages/mainline/ubuntu/ codename nginx
For Debian/Ubuntu then run the following commands:

apt-get update
apt-get install nginx


Per /etc/apt/sources.list:

## Uncomment the following two lines to add software from Ubuntu's
## 'extras' repository.
## This software is not part of Ubuntu, but is offered by third-party
## developers who want to ship their latest software.
# deb http://extras.ubuntu.com/ubuntu trusty main
# deb-src http://extras.ubuntu.com/ubuntu trusty main

deb http://nginx.org/packages/ubuntu/ trusty nginx
deb-src http://nginx.org/packages/ubuntu/ trusty nginx