To replace all occurences of a given character :

 String   myString = "Replace ' by * ";
String   tmpString = myString.replace( '\'', '*' );
System.out.println( "Original = " + myString );
System.out.println( "Result   = " + copyString );

To replace a character at a specified position :

 public static String replaceCharAt(String s, int pos, char c) {
   return s.substring(0,pos) + c + s.substring(pos+1);
}

To remove a character :

 public static String removeChar(String s, char c) {

   String r = "";

   for (int i = 0; i < s.length(); i ++) {
      if (s.charAt(i) != c) r += s.charAt(i);
   }

   return r;
}

To remove a character at a specified position:

 public static String removeCharAt(String s, int pos) {
   return s.substring(0,pos)+s.substring(pos+1);
}