How do I find my WEP or WAP password on Airport to allow a Sony PS3 to connect?
Check and see if this User Tip from forum expert Tesserax will do what you need:
AirPort - Regain Access from Forgotten Passwords
If not, you can perform a Soft Reset on the AirPort Express to reset the passwords. See this Apple support document for details:
Resetting an AirPort Base Station or Time Capsule FAQ
Retrieving data ...