Easiest way IMO? Go to your User CP, then go to Edit Signature. When you scroll down a little you'll see your tracker "code" -- It starts with [url=http://www.3fatchicks.com
Your weights are listed in this order in the "code": Starting/Goal/Current. Change the last weight, it will be the one that appears just before /.png. Just change the numbers, leave everything else. Clicking on Preview Signature at the bottom of the page will confirm you've done it correctly. When you have what you want, be sure to click Save Signature.
Like anything on this earth, it's much easier to do than tell