As per the rules, you can not change the priority order that you set during the first round. Only in the last spot round, candidates who have not been allocated any seats can change their preference order.

Well, last year JOSAA had a special seat allocation round for NITs/IIITs/GFTIs to fill in the seats left vacant even after the last round. In that round, candidates were allowed to change their initial priority order and re-fill the choices. But ofcourse, you can not rely on such uncertainities.

So dear you cannot modify choices now.Also it is advised to fill choices more and more if you just want any seat in IIT.But,if you haven't alloted a seat then please wait for upto the spot round.May you get a seat then.

Hope this helps.