Samsung is now rolling out a new software update to the Galaxy S8 and Galaxy S8+. The latest software update adds the December 2018 security patch, few bug fixes, and minor improvements.
The build version is G950FXXU4CRL3 and G955FXXU4CRL3 for the Galaxy S8 and S8+ respectively. The size is 425 MB.
If you want to check this update manually, head into Settings > Software update > Download updates manually or you can simply wait till the update automatically appears in your phone.