You can download the source code from http://odoo.com/page/download.  You have to login with the account related to your subscription, and also fill out a senseless form that asks questions like you are a new user of Odoo.    

After downloading the source, you can uncompress the source and update your installation.

Hope this helps.

First of all, as an enterprise customer, you should read the enterprise docs or ask Odoo. Most probably you should make an appropriate entry in the /etc/sources.list to update your installation, like it is for the community installer.