The most recent non-preview release of Android, version 5.1.1 Lollipop, began rolling out to Samsung's Galaxy S6 and S6 edge a couple of weeks ago, starting with those models sold by T-Mobile, followed by US Cellular a week later.
A new Lollipop update is now rolling out to the same handsets on Sprint - but it's not actually the latest version.
In fact, it's the slightly older Android 5.1 update - which made its first official appearance back in February - that's now making its way to the two Galaxy flagships on Sprint. Along with the various improvements that 5.1 brings, Sprint's release notes also refer to unspecified bug fixes included with its update.
The carrier points out that the update "may be released in stages and can take several days for delivery". This is pretty standard when it comes to update rollouts, so if you don't immediately see Android 5.1 available to download on your Galaxy S6 or S6 edge, don't panic; just check again a bit later.