How do I update my iPhone 6 to iOS 12.5.5

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

not enough storage space?

If you need more space for an update on your iPhone, iPad, or iPod touch - Apple Support

issues?

Updating iOS: common issues and solutions - Apple Community

video..

Teebanks101 Said:

"How do I update my iPhone 6 to iOS 12.5.5"

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Use a Computer and Ethernet:

Start with attempting an update through use of a computer. If you cannot use Wi-Fi, then use Ethernet.  See my User Tip: Update your iPhone Using a Computer - User Tip. On the computer, you would connect it through Ethernet, going directly into your router. For a Mac (or PC) that does not have Ethernet, this would be the adapter to use: Belkin USB-C to Gigabit Ethernet Adapter - Apple. If you get the same error, the model is likely just not compatible.

iPhone update