How can I update my ios

Please read through the user tip below, then post back with relevant information.

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Assuming you have a compatible device, see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support

If the above Apple articles do not help, please post back with relevant information. 

Check if your device has been jail break, as then in my opinion you won't be able to receive the official updates and then you have to do everything manually.

One of my cousin had an iPhone 7+ and he jail break the device and after that there were no official update from the apply that had been received by him.