Most of the US carriers have released the over the air (OTA) update to their customers who are now enjoying the Android 5.1. HTC just announced that the customers in EMEA - Europe, Middle East and Africa, are about to start seeing the update on August 4th, or today.
As we already know, 5.1 doesn't bring major changes, but it would feel good to know that you have the latest and greatest Android version on the phone. Currently HTC offers Android 5.0.2 with Sense 7 on its phones. LG G4 has had Android 5.1 for a few weeks now and Samsung Galaxy S6 is slowly getting the update across many territories.
HTC has publicly said that it will update this phone to Android M, but we will be lucky if that happens before the end of 2016. Traditionally, the latest and greatest Android launches in Q4. Judging from Lollipop Android 5.0 launch, we might expect Android M to role out in late October or November for Nexus devices and later for everyone else.