I assume you mean you want to change your password for the AS forums, like this Q+A page.
The user ID and password that you use on EBay are the same you use to place snipes here, since AS bids for you.
To change your user ID and/or password for these forums, go to the "MyPop" link just above these forums, and below the blue AS tabs. Click on PROFILE and you will see a screen for you to change your password and other information.