hi , 

I think the last date is over. The last day was extended upto 22-11-2018. However, you can still go to cbse home page and log in with your credentials like roll number and school number(if given) and then proceed for the submission of the improvement form. and pay online, they would tell you the late fee charges too. Refer their home page: hwww.cbse.nic.in/newsite/index.html - this is the home page and you get to log in . 

All the best. For any query , feel free to contact.