Firstly, you need to let us know what you want to change it on. If this is on your PC, then you will require instructions for this but if it on your router then we would have to know what make and model of router you have and also who your ISP is. By default, your router should be set to gain it's IP address automatically and will then apply settings for DNS from the ISP to the router. You would then have DHCP setup on your PC which in turn would gain its DNS from the Router. if your PC is set to static IP address, try to set it to DHCP and see how you get on.
failing that, let us knwo your PC operating system, router and ISP so we can better see what you need to do.
Adeyfk, January 2007