HomeKit thermostats and Celsius
There's no way to make that change directly in the Home app.  This issue was mentioned during the iOS13 beta testing and wasn't changed.

Some people have reported that as of iOS 13.1.3 you give a command to Siri and it will set the correct temp.  I'm in the US, thus don't use celsius, so I can't confirm this is correct.  You didn't mention which specific iOS 13 version you were using.

Apple's 13.1.3 release notes don't specifically call this out.  It's not unusual for Apple to fix things and not put them into the release notes.