connect to your device through Telnet or SSH
type Enable for configuration and type the enable password.
SW#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
SW(config)#interface vlan20
SW(config-if)#ip helper-address 192.168.0.254
SW(config-if)#end
type show running-config to check if your changes are applied.
to save type copy running-config startup-config