How to reset Wi Fi password when old one forgotten?

Perform a "soft reset" in accordance with the following excerpt from Resetting an AirPort Base Station or Time Capsule FAQ

1. Connect the AirPort Base Station or Time Capsule to power and wait for it to complete its start up process. To learn more about the start up process, please seeAll about Wi-Fi Base Station Status Lights (LED).
2. Press and hold the reset button with a pen or pencil for 1 full second, then release it. The light (LED) will begin flashing amber, indicating that the AirPort Base Station or Time Capsule is in soft reset mode.
3. From the AirPort menu bar item, choose the network created by the AirPort Base Station or Time Capsule (the network name does not change).
4. Open AirPort Utility in Applications > Utilities on a Mac using Mac OS X, and in Start > All Programs > AirPort on a computer using Windows.
5. Select the AirPort Base Station or Time Capsule and click Continue.
6. A window indicating that the device has been reset will appear.
7. Select "Restore the previous settings" and select OK.
8. Move through subsequent windows to make changes to your passwords or other settings as desired, clicking Continue after each one.
9. When you have moved through all of the selection windows you will be able to select Update. Click Update to install the new settings.
10. Click Continue to allow your AirPort Base Station or Time Capsule to restart.
11. Quit AirPort Utility.