Hey Vishnu.

The concept of 2nd and 3rd attempt in MUOET is that Manipal University petmits you to take the exam second and third time if you think you did not do well in the exam and you are not satisfied with your performance in the first or second attempt. For every additional attempt you will have to register yourself again and the procedure for it remains same as it was when you regiatered yourself fot the first attempt.  The best score out of all your scores will be considered for ranking and seat allocation.

Regarding the changing of the slot, once the candidate has booked a slot, it cannot be changed. You can, however, contact tge authorities if you have a very genuine problem and talk to them about it. 

Hope this helps. All the best.