Yes you can sit for the second round of counselling

Yes, you can fill in fresh choices and participate for upgradation in 2nd counselling. But, to be eligible, you have to report to the allotted college and join the college. When you report at the allotted college, you have to indicate your willingness to participate in the second round

Feel free to ask me in the comment box if you are having any further doubts

hope you find it helpful 

Good Luck!