It sounds like you're looking to send a password reset email. See this example from the Firebase documentation:

FirebaseAuth.getInstance().sendPasswordResetEmail("user@example.com")
    .addOnCompleteListener(new OnCompleteListener() {
        @Override
        public void onComplete(@NonNull Task task) {
            if (task.isSuccessful()) {
                Log.d(TAG, "Email sent.");
            }
        }
    });