Android M right now is only targeted towards developers and is still in development phase. Similar to last year, Google has already introduced Developer Preview images of Android M on multiple devices such as Nexus 5, 6 and 9. Unlike last year, Google plans to roll out monthly OTA (over-the-air) updates to users who will be using these Android M builds regularly on their Nexus devices.
Google has announced a timeline to release OTA updates for the Android M Developer Preview till July — and after that we could see the final release somewhere in Q3 2015. Last year with Android L Preview, Google only posted a single firmware image file for the supported Nexus devices and no updates were released later, till the final was announced.
You can check for updates like on any normal Android version. Go into Settings>About Phone>Software Updates.
Fortunately, the final build set to release in Q3 this year will be also available as an OTA for Developer Preview images. This was previously unmarked and developers had to manually flash the final images, leaving them to wiped phones every time.