How do I update my iPhone 6 software

You can't.

Unfortunately 3rd party apps requiring a more recent (beyond 12.5.5) operating system version, cannot run on your 8 years old device. I am sure you will not like this answer of mine, but there are no solutions, other than to buy a new(er) iPhone model.

Regards

Giulio

You can't, sorry. See why in this compatibility chart:

You're most welcome