To apply for CBSE Improvement Exam the Candidate who has passed the examination of  the  Board  in  2018  can  apply  in  2019  for improving the performance in one or more subjects.

The Candidate   who   has   passed   Class   XII   under vocational   stream in   2017 can   apply   for improving  the  performance  in  the  examination  to
be  held  in  2019  if he/she  has  not  availed  the chance in 2018.

You can check the detail with the given below link.

http://cbse.nic.in/newsite/attach/Final%20private%202019%20-%20Class%20XII.pdf

Good Luck!