Google has been slowly rolling out the new update to 6.0.1 to Nexus devices. The OTA has not yet hit every device. That being said you can flash the stock images which have all been available for about a week now. If you are flashing the update from Google but would like to keep save your data you can do so with the modified stock images from RootJunky. These images also have working flash-all.bat files meaning you can just double click a file to flash the stock rom to your device. The no wipe stock files have been available for the 5x and 6p for the past week. Today Rootjunky added the no wipe stock files for Nexus 5 and 9. Head to the link below for the download.
via rootjunkysdl.com