Upgrade iPhone to current iOS minor release versus going to the new major release version?
Good question. My guess is that you can do it if:
you use iTunes
Apple is signing an '.ipsw' file for your iPhone model.
If I was trying to do this, I would look at https://ipsw.me/product/iPhone.
They seem to have a good / easy system for figuring out what is available for which iPhone.
Update:
If you have a 6s, it does not look like that model is supported. At least that is what I gather from looking at https://ipsw.me/12.4.3.
Update 2:
https://iosindex.com/iphone also reminded me that there are at least two versions of the iPhone 6s, the "GSM" version and the "everyone else" version. Their pages for each are linked below:
iPhone 6s (GSM)
iPhone 6s (Global)