How can I change the contents of an EditTextPreference by code?

This is the settings_red.xml code

<!-- Usuario de la red -->
<EditTextPreference
    android:key="usuario_reducf"
    android:selectAllOnFocus="true"
    android:singleLine="true"
    android:summary="@string/pref_usuario_resumen"
    android:title="@string/pref_usuario_titulo"
    />
<!-- Password de red -->
<EditTextPreference
    android:inputType="textPassword"
    android:key="contraseña_reducf"
    android:selectAllOnFocus="true"
    android:singleLine="true"
    android:summary="@string/pref_contraseña_resumen"
    android:title="@string/pref_contraseña_titulo" />

This code will take it out of stackoverflow in English but it gives me null or it will not instantiate the editTextPreference

  // Reutilización del fragmento
        String settings = getArguments().getString("settings");
        if ("generales".equals(settings)) {
            addPreferencesFromResource(R.xml.settings_gen);
        } else if ("reducf".equals(settings)) {
            addPreferencesFromResource(R.xml.settings_red);

            user = (EditTextPreference) findPreference("ususario_reducf");
            password = (EditTextPreference)findPreference("contraseña_reducf");

            user.setText(getPreferenceScreen().getSharedPreferences().getString("username", ""));
            password.setText(getPreferenceScreen().getSharedPreferences().getString("password", ""));
        }

For them to understand better what happens is that at the beginning I ask for username and password to authenticate in a network and I want it to store it in sharedpreferences once it enters the configuration of the apk load the ones it has saved , so I leave blank. Any help?