you cannot change DNS IP thru GUI, CLI is the only option
Refer below command
set network dns
This command sets the IP address for the primary or secondary DNS server.
Command syntax
set network dns { primary | secondary } ip-address
http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/cli_ref/8_6_1/cli_ref_861.html#pgfId-39980