iPhone with constant crashes

Sudden battery drain on iPhone 6s Plus

Since I’ve up dated the phone to OS 13.6 on Weds, July 16, it has crashed to restart the OS 2 and 3 times a day. The OS has been re-installed 3 times in a week, the last on Thurs, July 23 evening. I reinstalled several of my Apps the same evening before the latest crash this morning.

Every time it crashes the battery level shows 10% or less. When it is charged it seems to reach 70 to 80 % very quickly and then takes a long time (several hours) to reach 100% .

The Battery Level shows the phone shut off during the night and restarted in the morning. The Battery usage shows the evening activities (App Store) and the Home & Lock Screen. The rest of the Activities (7) are all 6% and under. There is no reasonable explanation why it should crash.

The phone was checked by Apple Genius at the Apple Store on Tue, July 21 and the battery was declared to be in good condition ar 87% Maximum capacity. The phone was Erased of all content and settings, and the OS reinstalled at the store.

I know others have had these crashes lately, and I tried several of the suggestions for help prior to taking it in to the Apple Store. Nothing stopped the crashes.

My question is: Is there anyway to reinstall to OS 13.5 or .4?

Bob D:)