Something

 public String byttUt( String kilde, String ut, String inn )
	  {
		 String replacement = "";
		 String token;
		      
		 StringTokenizer st = new StringTokenizer(kilde);
		 while(st.hasMoreTokens()) 
		 {
			 token = st.nextToken();
			 if(token.equals(ut)) 
			 {
				 token = inn;
				 replacement += (token+" ");
			 }else 
			 {
				 replacement += (token+" ");
			 } 
		  }