If it's the Time Capsule psw, as opposed to Time Machine, that you've forgotten, the Setup Guide says that pressing the reset button for one second will allow you to
* select the AirPort network
* open AirPort Utility
* select the TC and choose Manual Setup from the Base Station menu
* click AirPort in the toolbar, then click Base Station
* enter a new psw for the TC
* click Wireless and choose an encryption method from the Security pop-up and set a psw for that
* click Update to restart TC and load the new settings