I recently had some issues getting NginX to update though apt. Here is what I did to fix the problem I was having.

W: GPG error: http://nginx.org wheezy Release: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY ABF5BD827BD9BF62

Run the following commands

wget http://nginx.org/packages/keys/nginx_signing.key
cat nginx_signing.key | sudo apt-key add –
apt-get update