Unlock Grandstream HT502

I changed my Voip provider and would like to use my Grandstream HT502 adapter with another voip provider but its locked. I tried to reset it but was unable to reset to factory settings. Could you please tell me how to reset to factory settings, where I can use the admin password as currently its locked by another password.

While not connected to the Internet, reset the unit (with reset button or phone keypad) and disable provisioning, as described in this thread.