You will be able to change it yourself when you have 30 posts AND have been registered for over 30 days. You've already got the 30 posts so all you have to wait for now is to get past the first 30 days. After that, on the same page where you put in the code for your signature, you will have a new option called Custom User Text:. That will be where you will change it. Before that time, if you have more than 75 posts, you will change over to "DIS Veteran".