What platform are you using? If you are using anything other than GoCentral changing the robots.txt is a matter of downloading it, editing it in a text editor (like Notepad ++) and uploading it. I'm not completely sure on the GoCentral side, but I do not think it can be changed. At least, I haven't found out how, but I am relatively new to learning the details of the GoCentral.
Hope this helps,