How to change a value of a file using Java IO function

If your configuration file is in the form of a Properties file. This means each line is in a format of key=value. You can easily read, modify and write it. Here is an example, I assume you have no problem with reading and writing a file through streams.

File sdcard = Environment.getExternalStorageDirectory();
File file = new File(sdcard,"sample.txt");
InputStream is = new FileInputStream(file); // Read your existing file
Properties props = new Properties();
props.load(is);
props.setProperty("wpa_oper_channel", "4");
OutputStream out = new FileOutputStream(file); // Your output file stream
prop.store(output, null); // Save your properties file without header

By doing this, you may lose if there is another type of line like comments or else.

Update

I updated to source code. But still you need to get read and write permission.

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

I haven't test the code, It will work with some exception handling. If it is not work please specify your error.