What exactly is this update doing?

Improving security and repairing defects (compared to iOS 13.5).

Restore your phone using your computer—>If you see an Update Failed or Apple Pay Not Available alert after iOS update or restart - Apple Support

Mine updated and now it has no service and is turning itself on and off. Says mobile data update failed and I can’t send or receive calls or texts. Phone is now useless

The worst part of this is the simple fix Is that it does not work. In the end you will have wasted hours of your time and get a 9006 error on your computer. The update is bugged. My phone thinks it’s got the newest update installed. The errors it flashes say it needs updating Anyone have a way to remove 13.5.1 update and get me back to a great phone?

Same. Turned my work phone into a great paperweight.