There's a critical exploit in the iOS Mail app that was just recently patched in iOS versions 13.5 and 12.4.7. (BTW, if you haven't updated your iOS version to either of these builds, you should disable the Mail app on your iPhone as an interim fix to avoid getting pwned.)
iOS 12.4.7 was supposedly pushed out for older devices that are incompatible with iOS 13.
I am using an iPhone 6s, running iOS 12.4.1. Checking for Software Updates via Settings app → General → Software Update only offers to install iOS 13.5.
I assume there must be some sort of download link where I can get an iOS 12.4.7 installer package and load it onto my phone via iTunes, although I wasn't able to find one.
Is there a way to install iOS 12.4.7 on iPhone 6s?
IMPORTANT: For anyone who didn't know, there's a big difference between the iPhone 6s and the original iPhone 6. The 6s has significantly improved hardware specs, most importantly double the RAM (https://www.theverge.com/2015/9/26/9401563/iphone-6s-teardown-ram-confirmed-2gb) which often makes the difference between "running the latest apps properly" and "puzzling, bizarre random crashes".
Best Answer
You are apparently out of luck here. While it's possible to download an iOS firmware (a .ipsw file) and install it onto a device, as long as Apple is signing is, Apple typically stops signing an older release a few days after issuing an update. IPSW is a well regarded website that lets you download all the available firmwares for a device.
However, Apple has not built iOS 12.4.7 for the devices that support running iOS 13 or later. Thus, iOS 12.4.7 was never released in the first place for iPhone 6s.
So technically speaking, there is no way to obtain and/or install iOS 12.4.7 for an iPhone 6s. Your only way to get patched for this particular vulnerability is by installing iOS 13.5 on your iPhone 6s.