How do I change dns server ?

I’m running on a NT 4.0 network with WINS, DHCP and DNS running. For Internet we use dns 198.6.1.4 and a second one 198.6.1.5 I called my T1 provider to let them know we could not get to some sites on the internet. He asked me to change my dns server because the ones I’m using are bogged down. How do I change them ? ?