How downgrade the Iphone 11 from IOS 13.4 to IOS 13

You could only downgrade to iOS 13.3.1. It is legal to do so but it isn't trivial and is not supported by Apple. You also have a limited window of opportunity to do so because Apple will soon stop signing iOS 13.3.1.

One thing that deggie didn’t mention: If you downgrade to 13.3.1 you will have to set up the phone as new. You will not be able to restore a backup made with 13.4.

If you say why you want to downgrade, if it’s because of a problem that you think is related to the version say what it is, there is probably a way to fix it.

I know that they are new and if you want to post a statement that is not correct that is up to you. I usually just tell them your second two sentences. They said they already looked on Youtube and there are lots of videos there showing you how to do it, some correct, some that really defy the imagination. And yes, the other Mr. Finch is correct, you can ONLY set up the iPhone as new, you can't use a backup.

Downgrading isn't supported by Apple.

You can't downgrade iOS. It is not supported by Apple. It has NEVER been supported by Apple. Do not trust YouTube videos on this, unless you'd like to chance bricking your iPhone.

Your first sentence is not true, Apple keeps a signed version of the previous one around for around 5 - 7 days for developers. Except for that your second and third sentences are correct.

I understand that. But the OP is new to Apple and didn't say they are a developer, so I stand by my comment for the vast majority of users. And it's now a week since iOS 13.4 was released.