How can I upgrade my phone

Alves004 wrote: "Too slow doesn’t download things fast and it hangs sometimes"

Alves ~ Does your iPhone 6 still have its original battery?

And what's the Maximum Capacity % here?...

Settings > Battery > Battery Health

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An iPhone 6 cannot be updated further than 12.5.5.

Which iOS version is currently installed on your iPhone 6?

If you are not sure.......

Settings > General > About > Software Version

If you see iOS 12.5.5, your iPhone cannot be updated