Usually this is NOT possible, but some wireless access points do allow it. What you have to do is log into the WAP using whatever userid and password you ahve setup and go to the wireless settings. There some routers will allow the password to be visible or hidden depending on some settings. If you router does not allow this, then you have to change the password to something new and be sure to RECORD that password someplace safe so you can retrieve it in case you forget in the future.
If you do not know the admin userid and password, try the defaults for your model router/WAP. You don't say what you ahve, so I can't be more specific with access instructions or default passwords.
If that doesn't work, then you have no choice but to reset the router/WAP to factory default settings and reconfigure it from scratch. This is usually accomplished by pressing and holding the Reset button on the rear of the device for some number of seconds, until the lights blink and the device reboots.