Log in to TheKnot, hover over My Knot, and click Account Settings. If you are having issues doing it that way, you can email
support@thebump.com and ask them to change your username. Give them a few suggestions just in case your top choice is taken. It may take up to 24 hours for the name to update.
See also: https://forums.thebump.com/discussion/12777979/is-your-username-knottie-here-s-how-to-change-it
Re: How to change your knottie8273552847463 username