Are you referring to a Pre Shared Key on the SSID or a Guest Account which is used to authenticate via the Captive Portal?
1) You can change the Pre Shared Key via the WLAN SSID Profile. Configuration > All Profiles > Wireless LAN > SSID
2) Or change the password of the User Account (if it is using the localdb). Configuration > Authentication > Servers > Internal DB.
You can offer a random password to a user if the account is manually created. If you wanted self-registration with a random password, you would need to consider Clearpass.