Android OS can be updated in various ways:
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Using the OTA method which involves using only the WiFi of your phone to download the firmware. This can be done through Settings > About Phone > Software Update. (Safe method)
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Using the PC Suite application released by the manufacturer for their phones. You connect the phone and update the phone if there is an update. (Safest method around)
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Using a ROM and a custom tool to directly flash the firmware onto the phone. (Not safe, but with wide area of customization in place)