During the postfix installation and configuration, I was presented with a few options on how to send and receive email. I might have chosen the wrong option. How do I go back to that configuration screen to change my option?
The postfix configuration is stored in /etc/postfix/main.cf and /etc/postfix/master.cf. You can change the options there and restart postfix. If you are going to be running a postfix server, you will want to become at least a little familiar with these two files (especially main.cf). The options are well documented at postfix.org.
As a tip, something I like to do is make a backup of main.cf then go through and delete everything that is commented out. That way I have less crap to sort through when I need to modify it and it makes it a little less overwhelming.