How to update Android OS manually?
Android OS can be updated in various ways:
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)
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)
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)