Cisco Set IP Helper on VLAN

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