How do I update my iPhone 6 Plus to iOS 13.4?

An iPhone 5s, iPhone 6, or iPhone 6 Plus isn't capable of 13 or above; they require at least 2GB of RAM.

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Angelica6273 wrote:

Icant download Facebook and sfotfy and instagram

If these are apps you want to be able to download, then it's time to retire your nearly 10 year old iPhone and buy a newer, more modern one, which runs a an iOS version Meta (not Apple) requires for their apps to run on an iPhone.