I thought I remembered how to do this, but I can't see it now. Zach turned 13 and I wanted to update my signature. Thanks!
At the top you should see "User CP" click on that, then scroll down to the section labeled, "Your Profile". There should be a link that says "Change Signature".
Good advice, Doug. Since I was here, I went ahead and changed it for you already, Jordan. Happy birthday to the new teenager!