Now that became an old story. Google starts releasing OTA (over the air) image files alongside full factory images. OTA images will not wipe your data and does not require unlocked bootloader for flashing. It means, you can flash latest Android version soon after the release of the new version without wiping your personal data and unlocking the bootloader.
Note that the OTA images are not incremental update; they are full firmware which you can sideload on your device via ADB (Android Debug Bridge) command. You do not need to unlock your bootloader, and your data should remain intact. However, Google recommend you to backup any important data you have on your device. To download the OTA images, you can always go to the Google Developers page [HERE] and download it for your device. Before downloading, you should agree with Google’s Terms & Conditions. OTA images for the latest builds (Android 6.0.1) are currently available for the Google Pixel C, Nexus 6P, Nexus 5X, Nexus 5, Nexus 6, Nexus 7 (2013), Nexus 9, and Nexus Player. (source)
