How do I change my IP address?

Reverse the action you took to change the IP Address. Follow these instructions on how to use DHCP:

OS X El Capitan: Choose a manual IP address or use DHCP

You don't need to know your ip address to use DHCP. If you want to use manual mode instead of DHCP ask whoever manages your network what ip address to use. In most cases DHCP will work unless specific action has been taken by the network administrator to restrict access to assigned ip addresses only.