Assuming its menu is similar to an RV110w, you can change it under Networking->WAN. However if you mean the DNS the router assigns to its clients then you have to change it under Networking->LAN->DHCP.
Assuming its menu is similar to an RV110w, you can change it under Networking->WAN. However if you mean the DNS the router assigns to its clients then you have to change it under Networking->LAN->DHCP.