Most of the system configuration such as changing the hostname, IP, DNS..etc can be done using VAMI interface (https://vCenterIPorFQDN:5480) however if you see the changes are not taking affect, e.g. changing the DNS IP then following command can be used.

Enable SSH using VAMI interface, select Access tab, click Edit and Enable SSH login.
Take SSH session via putty to connect vCenter Appliance.
Use following command and change required configuration change.

root@vcsa[~]# /opt/vmware/share/vami/vami_config_net
Main Menu
0)      Show Current Configuration (scroll with Shift-PgUp/PgDown)
1)      Exit this program
2)      Default Gateway
3)      Hostname
4)      DNS
5)      Proxy Server
6)      IP Address Allocation for eth0

Here are some samples:-

Review current configuration.

Enter a menu number [0]: 0
Network Configuration for eth0
IPv4 Address:   192.168.0.51
Netmask:        255.255.255.0
IPv6 Address:
Prefix:
Global Configuration
IPv4 Gateway:   192.168.0.1
IPv6 Gateway:
Hostname:       vcsa1.lab.org
DNS Servers:    127.0.0.1, 192.168.0.10
Domain Name:
Search Path:
Proxy Server:

Whenever you make any changes you will be prompted for following warning message.

Warning: if any of the interfaces for this VM use DHCP, the Hostname, DNS, and Gateway parameters will be overwritten by information from the DHCP server. Type Ctrl-C to go back to the Main Menu

Changing hostname

Enter a menu number [0]: 3
New hostname [vcsa1.lab.org]:vcsa2.lab.org

Changing DNS server and Domain name.

Enter a menu number [0]: 4
DNS Server 1 [127.0.0.1]: 192.168.0.10
DNS Server 2 (optional) [192.168.0.10]: 192.168.0.11
Domain Name (optional) []: lab.org
Search Path (space separated) (optional) []: lab.org
DNS server settings updated

Changing proxy address of vCenter appliance

Enter a menu number [0]: 5
Is an IPv4 proxy server necessary to reach the Internet? y/n [n]: y
Proxy Server (http:// will be auto prepended) []: proxy1.lab.org
Proxy Port []: 8080

Changing IP address.

Enter a menu number [0]: 6
Configure an IPv6 address for eth0? y/n [n]: n
Configure an IPv4 address for eth0? y/n [n]: y
Use a DHCPv4 Server instead of a static IPv4 address? y/n [n]: n
IPv4 Address [192.168.0.51]: 192.168.0.51
Netmask [255.255.255.0]: 255.255.255.0
IPv4 Address:   192.168.0.51
Netmask:        255.255.255.0

Is this correct? y/n [y]: y

Reconfiguring eth0...
net.ipv6.conf.eth0.disable_ipv6 = 1
Network parameters successfully changed to requested values