Hi @raziatul your master password is the password you receive when you first time sign up in steemit. Then all the other keys you get via your master key. You have to keep your master password somewhere safe offline. You only use it when you want to change all of your keys. You can use your private owner key to change other keys.

my question, if another key has been stolen. and the thief changed another key. can I take the key from the thief with the owner's key?

as long as your master password not stolen then you're ok. You can change your owner key with the master password.