How can I download iOS 10.3.4

iOS 10.3.4 is the latest iOS version for iPhone 5

What happens when you go to Settings -> General -> Software Update?

See the following Apple articles:

Update your iPhone, iPad, or iPod touch

Update your iPhone, iPad, or iPod touch - Apple Support (IN)

If your iPhone, iPad or iPod touch won’t update

If your iPhone, iPad, or iPod touch won't update - Apple Support

You can do the update via PC, see this Apple article: Update software on iPhone, iPad or iPod touch in iTunes on PC

Update software on iPhone, iPad or iPod touch in iTunes on PC – Apple Support (UK)

Bearing in mind that if your iPhone is a 5c model, that iOS version is not available (the latest supported is v10.3.3).

If it is a 5 model, go to Settings>General>Software Update and see if v10.3.4 is available, then download it.

Regards

Giulio