How to use match on String value with Scala?

I'm trying to iterate on a String value to change each occurence of it. For example i want that "1" become "one", "2" become "two", etc.

I've done this :

  override def toString = {  
    val mapXX = init.map(_.clone);  
    var returnVALUE = mapXX.map(_.mkString).mkString("\n")

    for(c <- returnVALUE){
        c match {
          case 1 => "one";
          case 2 => "two";
          ...
          case _ => "";
        }
    }
    returnVALUE
  }  
} 

It didn't change anything of my list, i have the same display of my list. Nothing has changed.

Did someone knows how can we iterate on each character of a String value in order to replace each character by something else ?

Thanks