1) Download the OTA.zip on your PC, rename the file update.zip, then place file in the same directory as adb. 2) Make sure you have USB Debugging checking in the Developer Options, then connect your phone. 3) Open the command prompt from the directory above on your PC and type the command: 'adb reboot recovery' 4) When in recovery, you’ll see an android laying down. Press volume up and release. The menu will appear. Select “apply update from adb” 5) Using the command prompt type: 'adb sideload update.zip' The file will load and the update will begin. 6) When completed, select reboot.
http://forum.xda-developers.com/google-nexus-5/general/info-nexus-5-ota-help-desk-t2523217