String nuevoString = viejoString.replace('c','m');
String palabra = new String("casa"); palabra = new String(palabra.substring(0,3)+"m"+palabra.substring(3));
cadena[2*i+1]=c; // c es caracter
stringItem1.setText(aux.toString().substring(0,2*i)+ c + aux.toString().substring((2*i+1),2*tam) );