There's a few separate issues. Let's discuss.
1) The Difficulty Setting can be lowered, but not raised. The ability to lower the Difficulty Setting was added by popular request, but it is a one-way change. There's an in-game warning about it, with a confirmation button.
2) Read @Gardenia
's post above. I -think- save files include the difficult setting so a previous save might be your best solution (quick-Save, Auto-save, Area-save, back-up save etc) . This is a good time for a reminder that having a back-up save is pretty standard in any long game. For Exiled Kingdoms, you can simple save to a new slow slot on the save screen (open the save menu, select on the of six save slots).
2) This forum is NOT the place to request a refund. Consider this a polite reminder about Forum Rules. Because this is not the place to discuss refunds, etc, I won't comment on the refund aspect.