Allow the user to change their Username and Password through the Settings screen (and, respectively, the PasswordChangeScreen, which should probably be named something else).
Have the changes reflected both in Firebase and in the App
Have the User confirm the change by typing their password twice (as is most cases with other application)
After the change is done, it should return them to the Settings screen
Allow the user to change their Username and Password through the Settings screen (and, respectively, the PasswordChangeScreen, which should probably be named something else).