If by the DNS address you mean the IP address of the DNS server that iPhone uses for queries in order to resolve the domain name that you provide, then this is not possible on the device without jailbreaking. The DNS server IP address is provided via iPhone via DHCP. Therefore, you will have to change the DHCP settings in the network configuration to which you are connecting, for example. Wi-Fi hotspot.
IP-, iPhone (, , DNS), DHCP . IP- iPhone DHCP , , DNS- .