How do I install iOS 13 for my iPhone 6?

anjamight ~ iPhone 6 is not compatible with iOS 13:

iOS 13 and iPadOS are compatible with these devices

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

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If no service see this Apple article:

If you see No Service or Searching on your iPhone or iPad

https://support.apple.com/en-us/HT201415

You can't update it, sorry. See why in this compatibility list:

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

iPhone 6 is an outdated iPhone (9 september 2014).

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