Hi there!!!

As you got 45% in your cbse board examination and now you want to repeat so yes you can give improvement examination as a private candidate . around month of October,form of improvement exam will come on cbse's website ,fill that and prepare for that,and give your exams again .The acceptance of marks varies from institution to institutions...

Hope you got your answer,do upvote it if it helped you...

Best of Luck for Future Endeavours!!!

Regards,
Rajdeep Sinha