Only through editor scripts. See AssetDatabase.SaveAssets and EditorUtility.SetDirty. Note that you can not call these functions from game scripts. It has to be in the editor. Otherwise you can check out this question which might be similar to your needs. It works on standalone players.
import System.IO;
var myString : String = "Write some text!";
var file : String = "somefile.txt";
Load();
function Load()
{
myString = File.ReadAllText(file);
}
function Save()
{
File.WriteAllText(file, myString);
}