It looks like HTC One M9 owners on Verizon and Sprint will be able to get the update first, starting April 10 (that's tomorrow). AT&T and T-Mobile One M9 variants should receive the update one week later, on April 17. It's not clear when users outside the US will get the update.
According to Phandroid, the update improves the One M9's camera capabilities in most scenarios, including when taking outdoor and indoor photos, macros, or photos with the LED flash on. You can see some sample photos (before and after the update) in the slideshow below. Apparently, the front-facing camera of the M9 doesn't need any fixes, and won't be affected by the update.
Available to order online since March, the HTC One M9 will hit shelves across the US tomorrow. The M9 costs $199 on contract, and, despite the fact that HTC just announced the Quad HD One M9+, it will remain the company's only flagship for North America (at least for the near future).