As you have appeared for the improvement examination being unsatisfied for your marks and if you have passed the 10+2 in 2021 then your improvement result will be held final and the previous result will not be regarded to be valid. Now for JEE ADVANCED the 75% age criteria is waived off, so you have no need to leave the dream of IIT, you are still eligible for it.

To know more about CBSE notification visit the link below:

https://www.india.com/wp-content/uploads/2021/08/CBSE-2.pdf

For JEE ADVANCED eligibility criteria visit the link below:

https://engineering.careers360.com/articles/jee-advanced-eligibility-criteria

I hope this answer helps. All the very best for your future endeavors!