I want iOS 13 for my iPhone

That is not possible for an iPhone 5S.

The highest possible for that model is iOS 12.5.5.

See: Update your iPhone, iPad, or iPod touch with iTunes - Apple Support

Picture10 wrote:

I want IOS 12 for my iphone 7 because 15 will not install- Where do I download IOS 12.5.5

You can not install iOS 12.5.5 on an iPhone 7.

What happens when you attempt to update to iOS 15.4?

What is the existing version of iOS?

I want IOS 12 for my iphone 7 because 15 will not install- Where do I download IOS 12.5.5

You don't.

Current IOS 11.4

Error Message

An error occurred installing (null)

Cant- computer running Windows 7 so cant't get latest version of itunes