Before you configure new DNS, It is adviced that be sure to write down the current   settings on a piece of paper. One thing I noticed that unlike the Open DNS it is very easy to remember.Google has given a detailed configuration settings here.

How to configure Google Public DNS on your Ubuntu?

The Google Public DNS IP addresses are as follows:
8.8.8.8
8.8.4.4

It is very easy to configure these DNS IP addresses on your Ubuntu.

Steps
1. Right-click on ‘Network Icon’ (located at top-right panel by default) and click on ‘Edit Connections’ to open Network Connections Manager.
2. Choose the type of connection you have. For this example, I will use ‘DSL’.
3. Under ‘DSL’, highlight ‘BSNL BB’ (my broadband connection) and click on ‘Edit’.
4. Inside ‘Editing BSNL BB’ window, click on ‘IPv4 Settings’ tab.
5. Under ‘IPv4 Settings’, change the ‘Method’ to Automatic (PPPoE) addresses only.
6. Put these nameserver addresses as your ‘DNS Servers’: 8.8.8.8, 8.8.4.4
7. Click ‘Apply’ and reboot your machine. You are done. Now your DNS has pointing to Google Public DNS.