Upgrade from iOS 12.4.1 to 12.4.7 on an iPhone 6s
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.