The simplest way would be to perform a "soft" reset on the AirPort Extreme. This type of reset will temporarily disable all of the Extreme's passwords up to five minutes. This should give you ample time to access the Extreme with the AirPort Utility to change it.
If you don't want to actually change the password, then you can possibly retrieve the password from the OS X Keychain, as follows:
- Run the Keychain Access utility found in \Applications\Utilities
- In the category column (on the left side of the utilities' window), select "All Items."
- In the right column look for your Network Name. It will have a "Kind" of "AirPort network password."
- Double-click on your network name listing.
- Click on "Show password."
- Enter your Mac's administrator's password
- Your wireless network's password should now be displayed.