HTC just posted the kernel source for One M9, which can only mean that a seasoned dev can tinker all he wants with HTC’s new flagship. You can take it from the dev site in the source link below and you should know it is based on Android 5.0.1, measuring around 500MB.
The download options include Asia, UK and a few other European country variants yet no US versions are available.
Usual users will not be too interested in this update because there’ s not much you can do with just these open source files. Developers can flash them to improve app functionality and work towards creating stable custom ROM builds for the device.