Passport date of birth discrepancy?

So my passport was issued 10 years ago. But we moved abroad and I didn’t had to renew it. Now that I want to I don’t have any Indian docs but I do have foreign residency docs, school papers and such.

I have a copy of my birthcertificate which isn’t in good condition but the hospital I was born in is now closed.

So how do I get my birthcertificate?

Given that I get my birthcertificate I need to apply for Indian passport but from what I know my date of birth is one year different from my birthcertificate and my expired passport. I have no idea why but my parents said it was to get me to admission earlier?! No idea as I don’t have the knowledge about such things. But I want help

How do I correct it? What are the steps? What do I need?