You can download iOS 15.7.2 (19H218) from here and use it on supported iDevices. This is currently compatible with the iPhone 6s and is being signed. That said, before downloading this IPSW firmware file, make sure to check the status below.
As you can only restore signed IPSW versions, only download this for updating if you see it indicated "Signed: Yes"
iOS 15.7.2 (19H218) for iPhone 6s
✓ This firmware is signed. This means that you can restore to it in iTunes.
Release Date 13th December 2022
Upload Date 2nd December 2022
Filesize 5.28 GB
iOS 15.7.2 Release Notes
This update provides important security fixes and is recommended for all users.
For information on the security content of Apple software updates, please visit this website: https://support.apple.com/kb/HT201222