However, many of the Nexus owners known to me (including me) were unable to get the OTA update on their phone while the rest of the people were getting it all over the world.
Of course, you need not always wait for OTA updates. You can download the official factory images corresponding to your Nexus device from Google's repository and flash your phone by using adb (Android Debug Bridge) and sideload command. But not everyone is familiar with adb and not everyone wants to follow all manual methods and update their phone, since OTA is just easy.
This post is not going to explain how to use adb and update your Nexus device with the latest Android Update. Here I'll explain a method which will tell you how you can make your phone to forcefully look for OTA update and continue with the Android 4.4.3 Update. These steps will work on all OTA updates (when your phone isn't notifying you with the update).
One important thing to remember is that you can only update to Android 4.4.3 if you are already on Android 4.4.2. Your phone should be connected to WiFi or 3G/4G network. Also make sure that your phone is charged enough so that it doesn't get switched off during the update process.
Step 1: Go to Settings > Apps > All tab > Google Services Framework
Step 2: Tap on Clear Data first and then tap on Force Stop (Tap OK on the next warning message)
Step 3: Restart your phone.
Step 4: If your phone shows an update notification on the Notification Bar, tap that and jump to Step 7.
Step 5: Go to Settings > About phone > System Updates
Step 6: Tap on Check now.
After this step, wait for few seconds and your phone would notify you that an System Update is available for your phone.
Step 7: Once you see the Update notification, Tap on Download to download the firmware to your phone. (The filesize for the 4.4.3 update for Nexus 5 is 92.2 MB)
Step 8: Once download is completed, you will be asked to Restart & Install.
This would take some time (approx 5-10 mins). Once restarted, you'll see that your phone has been updated to Android 4.4.3 KitKat.