Configuring the FreePBX Server Domain

Please how do I change the address of my FreePBX server to a domain for example

You are going to need to be a bit more clear. What do you mean change? Change where? If you want to be able to refer to your PBX by DNS name instead of IP you would need to register said domain in DNS and create an A record that corresponds to the IP address that’s assigned to your PBX.