How to increase iOS to iPhone

That may or may not be possible.

Update your iPhone, iPad, or iPod touch - Apple Support

You have given us no details so we don’t know.

Oh, you want to downgrade your iOS.

This is not possible. Like at all.

Whats the problem with installing an app on your current iOS version?

Gabrietta wrote:

I ... would like to increase iOS to 10.3

If you want to increase iOS, you may do that. However, you would have to increase to the current version of iOS (12.2 right now).