Google has started offering manual downloads of OTA update images. You can now download these files to your desktop and update your Nexus smartphone or tablet anytime by simply using the adb sideload command on PC. No need to download the full system image.
All the official OTA files can be downloaded from the below provided URL for Pixel C, Nexus 6P, Nexus 5X, Nexus 5, Nexus 6, Nexus 7 (2013), Nexus 9 and Nexus Player. The page has latest two OTA updates. You can choose the one you want to update on your device. Just remember, you must need to unlock the bootloader for flashing those files.