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This might be different between different phones. Most updates are found in Settings > About > Software Updates.
If you are not able to see any updates then your phone is not compatible with the newer version of android and you will not be able to get an official update.
You can get unofficial updates by installing a custom operating system on your device. However that process has the risk of bricking your device.
Why do you want to upgrade to android 4.4.4 when android when android 6 is available?
If your phone can support 4.4.4 you will get OTP from google directly if its doesn't then u have to flash it with custom firmware of 4.4.4
It depends on your phone manufacturer. If the 4.4.4 update is released by your manufacturer you can update it by going to settings and then about phone and then click software update. If you are a nexus phone owner you will get immediately once Google rollsout the update.
from settings and update its simple
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