sir i want ot give improvement exam as i got 81 percent in boards i was expecting 90 but dont know how i got this much ….......can i give improvement exam whil in collage first year

You can give improvement exam, but i would like to know, what would be the most benifit....you may be or may not be increase your marks, also there would be loss of 1 year education credentials.
81% is quiet good marks, just happy with it and try to get more in graduation assets...

Yes you can most of boards the boards have provision for doing so but rules are different for applying and all. If you are able to improve your score it can be great help in JEE Mains rank.