Hi

If one wants to improve his class 12th board examination marks then he or she can go for improvement examination

Now, improvement examination can be given only once and that also just in the year which comes after your year of passing class 12th board examination,

for example , if you have passed your class 12th board examination in 2020 then you are eligible for improvement examination only in the year 2021 and not after that.

So ,  if you have just found this year your 12 then you will be eligible for appearing in improvement examination but it has been more than one year of passing your 12th board examination then now you can't give improvement examination.

Also,

don't worry for competitive exam your 12th marks doesn't matter much even for examinations like UPSC, SSC CGL etc you just need to be a graduate in any discipline, there's no percentage barrier either in graduation or class 12th

Thank you