Dear student,

No it is not necessary to withdraw your seat from josaa 6th round counselling. If you have got your seat and accepted it then you can retain your seat or you can withdraw your seat, that is completely upon your choice. But if you got allotted any seat in iit then you cannot withdraw your seat.
Now if you want to take admission in your allotted college then you have to give the partial admission fees and accept all the terms and condition of the admitting institute or otherwise you can take part in Csab special round counselling.

Also check out the following link to know more about josaa counselling :-
https://josaa.nic.in/webinfo/File/GetFile?FileId=24&LangId=P