A simple solution: The read position is stored in the MBP file that has the same title as the book, you can find these by conecting your pc to the kindle and go to the documents folder in your kindle, look for the book title and you will find another file withe same name but if you check its properties you will find that it has an MBP extention. If you delete this file and go back to the book after ejecting the kindle you will find you are back at the start of the book
WARNING… if you have made any notes or alterations using the kindle to that book this info will be lost.
Hope this is an easy answer, it works for me and is straightforward.