App lost by updating to OS 13.1

Read this please:

Apple - Find My iPhone, iPad, Mac, and Apple Watch - http://www.apple.com/icloud/find-my-iphone.html

"The new [iOS13] Find My app combines Find My iPhone and Find My Friends into a single, easy-to-use app on iOS, iPadOS, and now macOS.* Find My can help you locate your devices and your friends and family. And now it can help you find your devices — even when they’re offline."

iOS 13 combined Find My Friends and Find My iPhone into a single app with the name "Find My." It should already be on your phone. Also see: Set up Find My on your iPhone and other devices - Apple Support

It's been combined with Find My iPhone and is now called Find My. Look for an icon that looks like this: