Cannot access field to change its modifier using java reflection

What im trying to do is:

Field f = /*some field from some class. the field modifier is "protected"*/
f.setAccessible(true);

But it says I cant access a field with "protected" modifier... So if I cant access it, how will I change the modifier?

Thanks!

EDIT:

((Field) getAllFields(nestedClass).get(1).get(nestedObj)).setAccessible(true); /*Exception is thrown because of this line*/
private ArrayList<Field> getAllFields(Class<?> c) {
    ArrayList<Field> fields = new ArrayList<>();
    do {
        fields.addAll(Arrays.asList(c.getDeclaredFields()));
        c = c.getSuperclass();
    } while (c != null);
    return fields;
}