My iphone 6s plus won't update.

I hate to break it to you, but that is not a 6s Plus. It's a 6 Plus. It can't run anything higher than 12.5.

Hayden ~ Please confirm that it is indeed an iPhone 6s Plus here:

Settings > General > About > Model Name

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You have 2 ways to update:

1. On the iPhone, open Settings, tap General and tap Software Update. You should see iOS 14.3 available as an update.
2. If this doesn't work, plug your iPhone to your Mac, open Finder, click your phone in the left pane, click Check for Update and follow the prompts.