How to update my iOS in my iPhone 6plus using my mobile data

You can only update the iOS using WI-FI. The cellular network will not work\ to update the iOS. You could update the iOS using your computer, but that would also require an ethernet or WI-FI connection to the computer.

Update your iPhone, iPad, or iPod touch - Apple Support

You can't. That requires 5G, which requires an iPhone 12.

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