Studying in Germany

I am in desperate need of assistance regarding my school leaving certificate. My biggest wish is to study Business Administration in Germany but I fear that my school leaving certificate does not meet the admission requirements to be accepted.
I would like to know if it is possible to attend a school where I can improve my grades, here in Germany. I finished school last year and I realised I messed up and desperately want to improve my grades to reach my life dream. Is it possible that I make an Abitur even though I already finished high school?
If not, what are my options on this subject.

Friendly Regards,
Janine Joubert