Samsung rolled out the Marshmallow update to the Snapdragon variant (SM-G900I) of the Galaxy S5 in India in the month of April, and the manufacturer is now making the update available for the Exynos version (SM-G900H).
As noted by SamMobile, the update is now available to all over the air or through Samsung's Smart Switch software on the PC. The update includes the May security patch, as well as all the new features introduced in Marshmallow.
Rocking the Exynos Galaxy S5? Head to Settings -> About device -> Software update to manually install the update.