That's a bit of a tricky one though I surmise that the easiest solution is to simply do a "save as" and assign a new password as you go, then delete the old version with the password you can't remember.

I'm a big fan of experimentation, however, so let's see what we can figure out...

First off, when you have a secure PDF here's what it looks like in the Finder:

Double click it, and you'll probably move into the Apple Preview app, which will show you a nice red screen:

The fact that it shows there are 30 pages to the document is a bit of a security glitch in my opinion: you should know nothing about an encoded/protected document until you validate the password, but that's a different story I suppose.

Enter the correct password and it'll show you the document "in the clear", without any privacy or protection. Just as you'd expect.

Now choose File -- > Save As...:

And the ensuing dialog gives you the option you need:

That's not too difficult, but let's also try the same thing in Adobe Reader, just out of curiosity...

Digging around, there's also no way to change the password on an existing PDF document that's been encrypted / secured, so apparently the best answer to your question of how to change the password on a PDF document is to use "save as" to specify a new password than delete the old one. Not elegant, but certainly functional.

One tip: when you do delete the old version, make sure you empty the trashcan (or, better, use secure delete -- switch to the Finder, then off the Finder menu you'll find "Secure Empty Trash...") to ensure that it's not sitting around forgotten on your system.