Update is mainly a security patch. It updates S8 and S8 Plus' security systems to June version. It also fixes a number of issues, including the faulty roaming and SMS features as well as a glitched Find My Mobile service.
Update weights a solid 270 megabytes. Its build numbers are G950USQU1AQFJ for S8 and G955USQU1AQFJ for S8+. It is available now, but may take a while to reach all the devices worldwide. Be patient - it will reach you sooner or later.