iphone 13's battery health dropped 12% in a spand of 2 months

(Fyi : i bought this phone from my aquaintance...

I suspect your acquaintance had a non-Apple battery installed. That's a typical decay pattern with too many cheap after-market batteries.

Usually, Apple will not service a device that has been opened by a non-Apple service provider, so the option of selling and then buying new directly from Apple or a major cellular carrier could be the better path.

We see many sad stories here from people who tried to save a little money by buying a used phone.

i just bought the phone and in the first month it dropped from 100% to 94%, in the 2nd month my phone just had software problems and later i had it fixed along with changing baseband ic board, and when i got the phone back, the battery health already dropped to 89% and in just 2 days it dropped again to 88%.

It sounds like you bought a used phone that had been "refurbished" by a 3rd party company. The battery readings very likely were artificially "boosted" in the rework process to make the battery condition appear much better than it really was.

Make an appointment at the Apple Store or take the phone to an authorized Apple service shop to have them check over and test the phone.

You can find a list of authorized service shops in your area using this link: Find Locations

i bought this phone from my aquaintance and i just checked that everything was alright before purchased it)

Obviously, everything is not alright with this phone. Try for a refund.

First off, when did you purchase the iPhone and where was it repaired, at an Apple Store or Apple Authorized Service Provider? The iPhone 13 was discontinued almost 1 year ago, so if you purchased it new it has been sitting in a box in a store for a while, which is not good for the battery health. Secondly, unauthorized repairs on your iPhone with non-genuine parts can give mis-readings.