In the game mario and luigi bowsers inside story how do you save the game?

Okay what I mean by this is how do you save it after you defeat dark Bowser? I have defeated dark Bowser multiple times but every time after i do it takes my to the credits then to the end scene and it restarts and I don't have a chance to save it! its really annoying! how do i stop that? how do i save it after i defeat him!? please help because it is really annoying.