How can I upgrade my iPhone 6 Plus to iOS 13.5

You can't. Your phone is too old, too underpowered and lacks the hardware required to go any higher than iOS 12.5.6. If you need a higher iOS version for an app, then you need to invest in a newer, more modern iPhone.

An iPhone 6 Plus isn't capable of 13 or above.

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You cannot.

The hardware is not capable of running anything newer than iOS 12 (the latest version of which is iOS 12.5.6.)

About iOS 12 Updates - Apple Support