Software Update

You haven’t provided enough information to suggest anything. But here are the requirements for software update:

Here are the conditions that must be met to update iOS:

• You must have a fast Wi-Fi connection; you cannot update over cellular data
• The phone can not be in Low Power mode
• The battery state of charge must be above 50%
• You must have at least 3 GB of free storage
• You cannot be connected through VPN
• If you are not planning for it to update automatically you should turn this option off, as it will only update when the phone is plugged in, connected to Wi-Fi and locked
• You should leave the phone plugged in until it finishes updating, both to assure that it remains above 50% and to keep Wi-Fi enabled (Wi-Fi turns off when the phone sleeps if not connected to power).

I also noticed that your phone is listed as a 6S with iOS 13. You can’t go back to an earlier version.

I also have an iPhone 6 and never had this problem... although I updated from 12.4.5.