How may I upgrade from ios12.5.5 to ios13 on my iPhone 6

It is not possible, iPhone 6 Plus can’t be update to a higher iOS.

iPhone 6 Plus is an outdated iPhone (9 september 2014).

iOS 12.5.5 is last security updates for iPhone 6 Plus, -> released September 23.

Paschal ~ Apple offers many newer iPhone models that are compatible with iOS 13 or higher:

iOS 13 and iPadOS are compatible with these devices

...found by searching Apple's Knowledge Base.

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PaschalTLP wrote:

How may I upgrade from iOS 12.5.5 to iOS 13. Some app installation require I have ios13

You need to buy a newer iPhone model to update iOS.