<?xml version="1.0" encoding="utf-8"?>

<PreferenceCategory android:title="@string/basic_settings" >
    <ListPreference
        android:defaultValue="70"
        android:entries="@array/listArray"
        android:entryValues="@array/listValues"
        android:key="updates_interval"
        android:persistent="true"
        android:summary="@string/SOME_SUMMARY"
        android:title="@string/SOME_TITLE" />
</PreferenceCategory>

Preference customPref = (Preference) findPreference("updates_interval");<-- your preferences key
 customPref.setSummary("desired string");

Preference etp = (Preference) findPreference("the_pref_key");
etp.setSummary("New summary");