sir maine apni 10th 2017-18 mai pass ho gya tha cbse se kya mai 2020 mai nios 1 subject mai improvement de sakta hu
No it is not possible as you have completed 12th by now. And improvement exams are not valid after this. You can give the improvement of class 12th if you want as they just happened recently and you can apply for the same.
Also if you think that you scored less in class 12 then there is a provision of revaluation and retotal. Sometimes students get an increase in marks after applying for this.
All the Best!