Design Entrance Exam

I would suggest visiting the IIIT Delhi website or even calling their admission office to know if there will be any late application or spot rounds available. Colleges sometimes run spot rounds when there are unused seats after holding the main rounds, so there is no harm in asking whether this option might be available. Also, approaching the admissions office could give you some information on how to go about it, even if it's not the usual route. If none of these prove to be successful, you could attempt preparing for next year's UCEED and making sure that you go through all the application processes well in advance so as not to miss out on any chances.