Last year's Android L preview changed little between its I/O announcement and Lollipop's November release. This will apparently not be the case with Android M—Google expects to push an OTA update to the developer preview about once per month until M is done.

This was mentioned in Google's developer documentation for the preview, but we checked with Google's Dave Burke to make sure. That's where we're getting the monthly time frame. Last year the minor L preview updates were only distributed as new images to be flashed, so this is especially cool.

If you go into the settings, the developer preview does indeed have an update menu, so I guess that's where you'll go when the OTAs come out. There will probably be updated system images too.

Source: Android Developers