Project-GC uses geocaching.com to authenticate, which means that you don't have a separate account here. When you come here, we redirect your browser to geocaching.com and ask them if you are who you claim to be, and get "yes" or "no" back. They check your password against your normal geocaching account and just tell us whether you succeeded or not. We never actually see or store your password.
So, if your question is how to change your password for both geocaching.com and Project-GC, the answer is that you do that at geocaching.com. If your question is how to get a different password here from geocaching.com, the answer is that it's not possible since you are using the same account on both sites.
