The DNS settings are in the router setup where you specify the LAN settings, and that is different in every make and model router.

Failing that,
  • in OS X you set the DNS server in System Preferences > Network > Advanced > DNS.
  • in iOS 9 Settings > WiFI > your network id > touch the "info" icon > DNS


"All you've got to do is own up to your ignorance
honestly, and you'll find people who are eager to
fill your head with information"
--Walt Disney