How can I update iPhone 6 Plus to iOS 13.8?

The iPhone 6 Plus can be updated to iOS 12.5.5, no further.

EDIT: As you seem to have already solved your tech support issue, could you provide the steps you utilized, as it may be a great help to others.

Dangthuy ~ Apple offers several newer iPhone models which will run iOS 13:

iOS 13 and iPadOS are compatible with these devices

...Found by searching Apple's Knowledge Base.

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