Changing DNS servers might increase the speed of your computer's domain name lookups, but it won't do much for your overall download/upload speed. If everything is going at 30kb/s, that's probably something to take up with your ISP.
If you do want to change your DNS settings in Ubuntu Linux, the key is in your /etc/resolv.conf file.
DNS Client Configuration
To configure your system to use DNS for name
resolution, add the IP addresses of the DNS servers that are
appropriate for your network in the file /etc/resolv.conf. You can
also add an optional DNS suffix search-lists to match your network
Below is an example of a typical configuration of /etc/resolv.conf for
a server on the domain "example.com" and using two public DNS servers.
> search example.com
> nameserver 18.104.22.168
> nameserver 22.214.171.124