It looks like the HTC One M9 isn't the only high-end device sold by Sprint to be receiving the update to Android 5.1 in this period. While HTC's flagship will get the new software tomorrow, the Samsung Galaxy S6 and S6 edge are already in the process of seeing it.
The rollout for them has started today. This comes after T-Mobile was the first US carrier to issue an Android 5.1 Lollipop upgrade to the Galaxy S6 edge. And after that happened, the update went on to become available in other parts of the world too.
For Sprint's handsets, the download is around 568MB in size, as you can see from the screengrabs above. Aside from bumping up the Android version to 5.1, the new software also brings with it the Device Protection feature, along with some unnamed (but obviously useful) stability improvements and bug fixes.
Furthermore, performance should have been enhanced as well, which is always a good thing. After applying the update on your Galaxy S6, it will be on software version G920PVPU2B0F7, while its curved screen touting sibling will list G925PVPU2B0F7 in its About section.