Current company blackmailing for the fake marksheets submitted

Hi, I was working with a startup before my current employment. There, I had submitted photocopies of tampered marksheets of my graduation. After working there for some months, I decided to start fresh and got a job with another startup. Here, I had submitted all original photocopies realizing that mistake I had made. During background check, the current employers found out that i had submitted false photocopies but didn't do anything about it. Now, after working here for some years, I have put my resignation in order to pursue better opportunity. Now, they have started dropping hints that they know about the mistake I have made. I am not certain as to what to do as I don't want my professional career to end and don't want to bring shame to my family. What legal options do I have?