I have konica minolta bizhub pro c500 machine, that is password protected, I don't know the password. how can i reset password?

You dont specify what you want the password to. So here are some options to try.

If you want admin access to the Fiery print controller the default password is "Fiery.1"

The default operator password for a Creo server is "colorserver"

I'm not positive about the default admin password for the C500 but I'd try these.
eight zeros: "00000000"
or: "12345678"

If these don't work chances are someone has changed them from the default. On the Fiery or Creo this usually means a software reload. On the machine a service rep can probably reset the admin password.