to correct this mistake, You have to follow some precautions.
First of all send that challan and zerox copy of your IITJEE form to IIT DELHI and have a zerox of all document.
Parallely send a mail to JEE, Delhi and informed about this mistake. you will have also send a details of your new letter which you will send by speed post(Not by courier). And in mail ,attach also a scan copy of your challan so that it will reach IITJEE office by mail and post.
you should use same mail ID as you had indicated in Your Jee form.
Do this as soon as possible