The ReRa1n tool can downgrade iOS on Windows and linux. ReRa1n is officially supported on Windows and quickly downgrades Apple devices from A5 chip to A11
You are in iOS 13 GM (Gold Master), wondering how to exit GM / beta to get official. this is a basic manual on a way to downgrade to be prepared for updating to a new edition
First, please back up all device data to prevent possible unfortunate circumstances. On iPhone, please access Settings> iCloud Account> iCloud> Backup iCloud (Setting> Apple ID> iCloud> iCloud backup) and turn it on to make backup.