In order to access to the Unleashed Management page to then change credential and created a new SSID/WIF ID, you need to know at lease the admin credential, so you can ask for those to the previous owner or perform a factory reset and create all from zero, let also add some options.
1:If you don't know the credentials of the SSID/Wi-Fi ID or the admin access, you can connect a computer to the current Wi-Fi of the ISP router or even directly to the Switch (like all are in the same subnet will work), once you are online access to the next website unleashed.ruckuswireless.com and try the next credentials, then you can change the SSID and admin credential.
2: Not, if the credential is not working, the factory reset will be the only option.
A:Disconnect all the Access Point and leave only one.
B:Perform the factory reset to that Access Point and the set-up, then you can connect the other access Pointy that will join automatically.
Physical Factory Reset an Access Points Ruckus
How to factory reset or factory default different type of Ruckus Access points? (CommScope Guide)
Ruckus Unleashed Set-up using your Mobile
RUCKUS UNLEASHED SETUP IN 5 MINUTES (tutorial video)
Technical Support Engineer | L2 TAC Wired