Is it possible to update iOS on my iPhone 6 Plus?

An iPhone 5s, iPhone 6, or iPhone 6 Plus isn't capable of 13 or above; they require at least 2GB of RAM. If you're trying to install 12.5.7, describe what happens in this thread.

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Your iPhone 6 Plus can’t get more updates if your iPhone its on iOS 12.5.7 that’s the last update for that iPhone modelo and isn’t compatible with iOS 13 later versions