Surprise! Samsung is now rolling first One UI beta update to the Galaxy S8/S8+ users that bring several features improvements. It includes the January 2019 security patch. The update is now available in India.
The update is currently rolling out with build version G950FXXU4ZSA5 and G955FXXU4ZSA5, for the Galaxy S8 and Galaxy S8+ respectively. While the size of the update is 1606.15 MB.
Once signed up, go to Settings and then Update Software to select the Download Updates Manually option. This will allow the software to update and load the beta version.