How can I update my software in my iPhone 6 to 13.5?

An iPhone 6 is not on the iOS 13 compatibility list. The iPhone 6s is, but not the 6.

But, if your iPhone model is compatible, then this article should help...

https://support.apple.com/en-us/HT204204

You can't. An iPhone 6 doesn't have enough RAM to run iOS 13 or above.

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If your watch is a Series 0 model, you can.

If your watch is a Series 1 through 4 model, it'll only pair if it's on a watchOS version prior to 6.0. If needed, contact Apple.

If your watch is a Series 5 model, you can't.

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