downgrading

My iPhone 6s plus was working just fine and then the update from 14.x to 15.5 completely messed it up. I can't use it without it being hooked up to power or it just keeps trying to unsuccessfully restart. The second i unplug it from power it eventually tries to restart over and over again.

I didn't update my phone for the longest while so now I can't downgrade to any version of 14.x because Apple only verifies the previous update.

My question is, is there a process i can follow to force the downgrade or can the Apple store do it for me?