Method 1:
Type the following command:
dig @8.8.8.8 example.com
You can replace 8.8.8. example.com with nameserver address.
e.g. dig @ns2579.hostgator.com
This should update the DNS and you'll be able to see website updated (if there is any problem or was parked page).
Method 2:
Ubuntu 12.04 does cache DNS using dnsmasq.
Use the following to clear the cache:
sudo kill -HUP $(pgrep dnsmasq)
This will also do the same thing like method 1. Both these command line options work for the ubuntu and you can use either one of them.