Greetings!

1. A candidate, who has already accepted a seat allotted by JoSAA 2017, can withdraw the seat by reporting at a reporting center before the last round of seat allocation (i.e., if JoSAA conducts a total of seven rounds of seat allocation, candidates can withdraw up until during sixth round of seat acceptance period). There is no option of withdraw of an accepted/allotted seat after the announcement of last round of seat allocation.
2. Candidates who accept the allocated seat in any IIT by reporting at the admitting institute or a reporting center but do not withdraw seat, irrespective of whether they attend classes or not, become ineligible for JEE (Advanced) in subsequent years.

I hope this was helpful. Thank you!