Owners of Developer Edition phones are the most dedicated of Android enthusiasts, they who eschew the traditional carrier model and put their money where their mouths are for the sake of total control and the principles of open-source operating systems. (Also, I suppose a few of them might be actual software developers. You never know.) For those who've purchased the Developer Edition of HTC's 2015 flagship, the update to Android 5.1 is going out right now.
The Developer Edition is the second version of the M9 to get Lollipop 5.1, assuming that Sprint started sending out the promised OTA earlier this week. One of the Android Police team has the Developer Edition and has already seen the alert for the over-the-air update - it's possible that so few of these specialized phones are sold that HTC is sending out the updates all in one batch. In addition to the tweaks and improvements to Android, the update reportedly adds improvements to data performance, call reception, camera performance, battery life, and the much-needed thermal management on the One M9.
The update is just shy of 800MB, so grab some Wi-Fi if you want to get it quickly. Remember that on Android 5.1 and later, any modification to the /system partition will result in failed OTA flashes... not that that would deter Developer Edition owners for too long.